History
Adobe Systems made the PDF specification available free of charge in 1993. In the early years PDF was popular mainly in desktop publishing workflows, and competed with a variety of formats such as DjVu,Technical details
A PDF file is often a combination of vector graphics, text, andPostScript language
PostScript is aif
statements and loop
commands. PDF is largely based on PostScript but simplified to remove flow control features like these, while graphics commands equivalent to lineto
remain.
Historically, the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that are output by the PostScript code are collected and File format
A PDF file is organized using%PDF-1.7
. The format is a subset of a COS ("Carousel" Object Structure) format. A COS tree file consists primarily of ''objects'', of which there are nine types:
* Boolean values, representing ''true'' or ''false''
* Real numbers
* (...)
) or represented as hexadecimal within single angle brackets (<...>
). Strings may contain 8-bit characters.
* Names, starting with a forward slash (/
)
* ../code>)
* Dictionaries, collections of objects indexed by names enclosed within double angle brackets (<<...>>
)
* Streams, usually containing large amounts of optionally compressed binary data, preceded by a dictionary and enclosed between the stream
and endstream
keywords.
* The null
Null may refer to:
Science, technology, and mathematics Computing
* Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value
* Null character, the zero-valued ASCII character, also designated by , often use ...
object
Furthermore, there may be comments, introduced with the percent sign (%
). Comments may contain 8-bit characters.
Objects may be either ''direct'' (embedded in another object) or ''indirect''. Indirect objects are numbered with an ''object number'' and a ''generation number'' and defined between the obj
and endobj
keywords if residing in the document root. Beginning with PDF version 1.5, indirect objects (except other streams) may also be located in special streams known as ''object streams'' (marked /Type /ObjStm
). This technique enables non-stream objects to have standard stream filters applied to them, reduces the size of files that have large numbers of small indirect objects and is especially useful for ''Tagged PDF''. Object streams do not support specifying an object's ''generation number'' (other than 0).
An index table, also called the cross-reference table, is located near the end of the file and gives the byte offset of each indirect object from the start of the file. This design allows for efficient random access to the objects in the file, and also allows for small changes to be made without rewriting the entire file (''incremental update''). Before PDF version 1.5, the table would always be in a special ASCII format, be marked with the xref
keyword, and follow the main body composed of indirect objects. Version 1.5 introduced optional ''cross-reference streams'', which have the form of a standard stream object, possibly with filters applied. Such a stream may be used instead of the ASCII cross-reference table and contains the offsets and other information in binary format. The format is flexible in that it allows for integer width specification (using the /W
array), so that for example, a document not exceeding 64 KiB in size may dedicate only 2 bytes for object offsets.
At the end of a PDF file is a footer containing
* The startxref
keyword followed by an offset to the start of the cross-reference table (starting with the xref
keyword) or the cross-reference stream object, followed by
* The %%EOF
end-of-file
In computing, end-of-file (EOF) is a condition in a computer operating system where no more data can be read from a data source. The data source is usually called a file or stream.
Details
In the C standard library, the character reading functi ...
marker.
If a cross-reference stream is not being used, the footer is preceded by the trailer
keyword followed by a dictionary containing information that would otherwise be contained in the cross-reference stream object's dictionary:
* A reference to the root object of the tree structure, also known as the ''catalog'' (/Root
)
* The count of indirect objects in the cross-reference table (/Size
)
* Other optional information
Within each page, there are one or multiple content streams that describe the text, vector and images being drawn on the page. The content stream is stack-based, similar to PostScript.
There are two layouts to the PDF files: non-linearized (not "optimized") and linearized ("optimized"). Non-linearized PDF files can be smaller than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the PDF file. Linearized PDF files (also called "optimized" or "web optimized" PDF files) are constructed in a manner that enables them to be read in a Web browser plugin without waiting for the entire file to download, since all objects required for the first page to display are optimally organized at the start of the file. PDF files may be optimized using Adobe Acrobat software or QPDF
QPDF is both a software library and a free command-line program that can convert one PDF file to another equivalent PDF file. It is capable of performing transformations such as linearization (also known as web optimization or fast web viewing), ...
.
Imaging model
The basic design of how graphics are represented in PDF is very similar to that of PostScript, except for the use of transparency, which was added in PDF 1.4.
PDF graphics use a device-independent Cartesian coordinate system to describe the surface of a page. A PDF page description can use a matrix to scale, rotate, or skew graphical elements. A key concept in PDF is that of the ''graphics state'', which is a collection of graphical parameters that may be changed, saved, and restored by a ''page description''. PDF has (as of version 2.0) 25 graphics state properties, of which some of the most important are:
* The ''current transformation matrix'' (CTM), which determines the coordinate system
* The '' clipping path''
* The '' color space''
* The '' alpha constant'', which is a key component of transparency
*'' Black point compensation'' control (introduced in PDF 2.0)
Vector graphics
As in PostScript, vector graphics in PDF are constructed with ''paths''. Paths are usually composed of lines and cubic Bézier curves, but can also be constructed from the outlines of text. Unlike PostScript, PDF does not allow a single path to mix text outlines with lines and curves. Paths can be stroked, filled, fill then stroked, or used for clipping
Clipping may refer to:
Words
* Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement"
* Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel
* Clipping (publications) ...
. Strokes and fills can use any color set in the graphics state, including ''patterns''. PDF supports several types of patterns. The simplest is the ''tiling pattern'' in which a piece of artwork is specified to be drawn repeatedly. This may be a ''colored tiling pattern'', with the colors specified in the pattern object, or an ''uncolored tiling pattern'', which defers color specification to the time the pattern is drawn. Beginning with PDF 1.3 there is also a ''shading pattern'', which draws continuously varying colors. There are seven types of shading patterns of which the simplest are the ''axial shading'' (Type 2) and ''radial shading'' (Type 3).
Raster images
Raster images
upright=1, The Smiley, smiley face in the top left corner is a raster image. When enlarged, individual pixels appear as squares. Enlarging further, each pixel can be analyzed, with their colors constructed through combination of the values for ...
in PDF (called ''Image XObjects'') are represented by dictionaries with an associated stream. The dictionary describes the properties of the image, and the stream contains the image data. (Less commonly, small raster images may be embedded directly in a page description as an ''inline image''.) Images are typically ''filtered'' for compression purposes. Image filters supported in PDF include the following general-purpose filters:
* ''ASCII85Decode'', a filter used to put the stream into 7-bit ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
,
* ''ASCIIHexDecode'', similar to ASCII85Decode but less compact,
* ''FlateDecode'', a commonly used filter based on the deflate algorithm defined in (deflate is also used in the gzip
gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and i ...
, PNG, and zip file formats among others); introduced in PDF 1.2; it can use one of two groups of predictor functions for more compact zlib/deflate compression: ''Predictor 2'' from the TIFF 6.0 specification and predictors (filters) from the PNG specification (),
* ''LZWDecode'', a filter based on LZW Compression; it can use one of two groups of predictor functions for more compact LZW compression: ''Predictor 2'' from the TIFF 6.0 specification and predictors (filters) from the PNG specification,
* ''RunLengthDecode'', a simple compression method for streams with repetitive data using the run-length encoding algorithm and the image-specific filters,
* ''DCTDecode'', a lossy filter based on the JPEG standard,
* ''CCITTFaxDecode'', a lossless
Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statistic ...
bi-level (black/white) filter based on the Group 3 or Group 4 Group 4 may refer to:
*Group 4 element, chemical element classification
*Group 4 (racing), classification for cars in auto racing and rallying
* G4S, formerly Group 4 Securicor, a prominent British security company
*IB Group 4 subjects
The Group 4 ...
CCITT (ITU-T) fax
Fax (short for facsimile), sometimes called telecopying or telefax (the latter short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer o ...
compression standard defined in ITU-T T.4 and T.6,
* ''JBIG2Decode'', a lossy or lossless bi-level (black/white) filter based on the JBIG2 standard, introduced in PDF 1.4, and
* ''JPXDecode'', a lossy or lossless filter based on the JPEG 2000 standard, introduced in PDF 1.5.
Normally all image content in a PDF is embedded in the file. But PDF allows image data to be stored in external files by the use of ''external streams'' or ''Alternate Images''. Standardized subsets of PDF, including PDF/A and PDF/X, prohibit these features.
Text
Text in PDF is represented by ''text elements'' in page content streams. A text element specifies that ''characters'' should be drawn at certain positions. The characters are specified using the ''encoding'' of a selected ''font resource''.
A font object in PDF is a description of a digital typeface. It may either describe the characteristics of a typeface, or it may include an embedded ''font file''. The latter case is called an ''embedded font'' while the former is called an ''unembedded font''. The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant CFF), TrueType
TrueType is an outline font standard developed by Apple in the late 1980s as a competitor to Adobe's Type 1 fonts used in PostScript. It has become the most common format for fonts on the classic Mac OS, macOS, and Microsoft Windows operating ...
, and (beginning with PDF 1.6) OpenType. Additionally PDF supports the Type 3 variant in which the components of the font are described by PDF graphic operators.
Fourteen typefaces, known as the ''standard 14 fonts'', have a special significance in PDF documents:
* Times
Time is the continued sequence of existence and events, and a fundamental quantity of measuring systems.
Time or times may also refer to:
Temporal measurement
* Time in physics, defined by its measurement
* Time standard, civil time speci ...
(v3) (in regular, italic, bold, and bold italic)
* Courier
A courier is a person or organisation that delivers a message, package or letter from one place or person to another place or person. Typically, a courier provides their courier service on a commercial contract basis; however, some couriers are ...
(in regular, oblique, bold and bold oblique)
* Helvetica
Helvetica (originally Neue Haas Grotesk) is a widely used sans-serif typeface developed in 1957 by Swiss typeface designer Max Miedinger and Eduard Hoffmann.
Helvetica is a neo-grotesque design, one influenced by the famous 19th century (1890s) ...
(v3) (in regular, oblique, bold and bold oblique)
* Symbol
* Zapf Dingbats
ITC Zapf Dingbats is one of the more common dingbat typefaces. It was designed by the typographer Hermann Zapf in 1978 and licensed by International Typeface Corporation.
History
In 1977, Zapf created about 1000 (or over 1200 according to Lino ...
These fonts are sometimes called the ''base fourteen fonts''. These fonts, or suitable substitute fonts with the same metrics, should be available in most PDF readers, but they are not ''guaranteed'' to be available in the reader, and may only display correctly if the system has them installed. Fonts may be substituted if they are not embedded in a PDF.
Within text strings, characters are shown using ''character codes'' (integers) that map to glyphs in the current font using an ''encoding''. There are several predefined encodings, including ''WinAnsi'', ''MacRoman'', and many encodings for East Asian languages and a font can have its own built-in encoding. (Although the WinAnsi and MacRoman encodings are derived from the historical properties of the Windows and Macintosh operating systems, fonts using these encodings work equally well on any platform.) PDF can specify a predefined encoding to use, the font's built-in encoding or provide a lookup table of differences to a predefined or built-in encoding (not recommended with TrueType fonts). The encoding mechanisms in PDF were designed for Type 1 fonts, and the rules for applying them to TrueType fonts are complex.
For large fonts or fonts with non-standard glyphs, the special encodings ''Identity-H'' (for horizontal writing) and ''Identity-V'' (for vertical) are used. With such fonts, it is necessary to provide a ''ToUnicode'' table if semantic information about the characters is to be preserved.
Transparency
The original imaging model of PDF was, like PostScript's, ''opaque'': each object drawn on the page completely replaced anything previously marked in the same location. In PDF 1.4 the imaging model was extended to allow transparency. When transparency is used, new objects interact with previously marked objects to produce blending effects. The addition of transparency to PDF was done by means of new extensions that were designed to be ignored in products written to PDF 1.3 and earlier specifications. As a result, files that use a small amount of transparency might view acceptably by older viewers, but files making extensive use of transparency could be viewed incorrectly by an older viewer.
The transparency extensions are based on the key concepts of ''transparency groups'', ''blending modes'', ''shape'', and ''alpha''. The model is closely aligned with the features of Adobe Illustrator
Adobe Illustrator is a vector graphics editor and design program developed and marketed by Adobe Inc. Originally designed for the Apple Macintosh, development of Adobe Illustrator began in 1985. Along with Creative Cloud (Adobe's shift to month ...
version 9. The blend modes
A blend is a mixture of two or more different things or substances; e.g., a product of a mixer or blender.
Blend
Blend may also refer to:
* Blend word, a word formed from parts of other words
* ''Blend'' (album), a 1996 album by BoDeans
* B ...
were based on those used by Adobe Photoshop at the time. When the PDF 1.4 specification was published, the formulas for calculating blend modes were kept secret by Adobe. They have since been published.
The concept of a transparency group in PDF specification is independent of existing notions of "group" or "layer" in applications such as Adobe Illustrator. Those groupings reflect logical relationships among objects that are meaningful when editing those objects, but they are not part of the imaging model.
Additional features
Logical structure and accessibility
A "tagged" PDF (see clause 14.8 in ISO 32000) includes document structure and semantics information to enable reliable text extraction and accessibility. Technically speaking, tagged PDF is a stylized use of the format that builds on the logical structure framework introduced in PDF 1.3. Tagged PDF defines a set of standard structure types and attributes that allow page content (text, graphics, and images) to be extracted and reused for other purposes.
Tagged PDF is not required in situations where a PDF file is intended only for print. Since the feature is optional, and since the rules for Tagged PDF were relatively vague in ISO 32000-1, support for tagged PDF amongst consuming devices, including assistive technology (AT), is uneven as of 2021. ISO 32000-2, however, includes an improved discussion of tagged PDF which is anticipated to facilitate further adoption.
An ISO-standardized subset of PDF specifically targeted at accessibility, PDF/UA, was first published in 2012.
Optional Content Groups (layers)
With the introduction of PDF version, 1.5 (2003) came the concept of Layers. Layers, or as they are more formally known Optional Content Groups (OCGs), refer to sections of content in a PDF document that can be selectively viewed or hidden by document authors or viewers. This capability is useful in CAD drawings, layered artwork, maps, multi-language documents, etc.
Basically, it consists of an Optional Content Properties Dictionary added to the document root. This dictionary contains an array of Optional Content Groups (OCGs), each describing a set of information and each of which may be individually displayed or suppressed, plus a set of Optional Content Configuration Dictionaries, which give the status (Displayed or Suppressed) of the given OCGs.
Encryption and signatures
A PDF file may be encrypted
In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can deci ...
, for security, in which case a password is needed to view or edit the contents. PDF 2.0 defines 256-bit AES encryption as standard for PDF 2.0 files. The PDF Reference also defines ways that third parties can define their own encryption systems for PDF.
PDF files may be digitally signed, to provide secure authentication; complete details on implementing digital signatures in PDF is provided in ISO 32000-2.
PDF files may also contain embedded DRM
DRM may refer to:
Government, military and politics
* Defense reform movement, U.S. campaign inspired by Col. John Boyd
* Democratic Republic of Madagascar, a former socialist state (1975–1992) on Madagascar
* Direction du renseignement milita ...
restrictions that provide further controls that limit copying, editing or printing. These restrictions depend on the reader software to obey them, so the security they provide is limited.
The standard security provided by PDF consists of two different methods and two different passwords: a ''user password'', which encrypts the file and prevents opening, and an ''owner password'', which specifies operations that should be restricted even when the document is decrypted, which can include modifying, printing, or copying text and graphics out of the document, or adding or modifying text notes and AcroForm fields. The user password encrypts the file, while the owner password does not, instead relying on client software to respect these restrictions. An owner password can easily be removed by software, including some free online services. Thus, the use restrictions that a document author places on a PDF document are not secure, and cannot be assured once the file is distributed; this warning is displayed when applying such restrictions using Adobe Acrobat software to create or edit PDF files.
Even without removing the password, most freeware or open source PDF readers ignore the permission "protections" and allow the user to print or make copy of excerpts of the text as if the document were not limited by password protection.
Beginning with PDF 1.5, Usage rights (UR) signatures are used to enable additional interactive features that are not available by default in a particular PDF viewer application. The signature is used to validate that the permissions have been granted by a bona fide granting authority. For example, it can be used to allow a user:
* To save the PDF document along with a modified form and/or annotation data
* Import form data files in FDF, XFDF, and text (CSV/TSV) formats
* Export form data files in FDF and XFDF formats
* Submit form data
* Instantiate new pages from named page templates
* Apply a digital signature to existing digital signature form field
* Create, delete, modify, copy, import, and export annotations
For example, Adobe Systems grants permissions to enable additional features in Adobe Reader, using public-key cryptography. Adobe Reader verifies that the signature uses a certificate from an Adobe-authorized certificate authority. Any PDF application can use this same mechanism for its own purposes.
Under specific circumstances including non-patched
Patched (Ptc) is a conserved 12-pass transmembrane protein receptor that plays an obligate negative regulatory role in the Hedgehog signaling pathway in insects and vertebrates. Patched is an essential gene in embryogenesis for proper segme ...
systems of the receiver, the information the receiver of a digital signed document sees can be manipulated by the sender after the document has been signed by the signer.
PAdES
PAdES (''PDF Advanced Electronic Signatures'') is a set of restrictions and extensions to PDF and ISO 32000-1 making it suitable for advanced electronic signatures. This is published by ETSI as EN 319 142.
Description
While PDF and ISO 32000- ...
(''PDF Advanced Electronic Signatures'') is a set of restrictions and extensions to PDF and ISO 32000-1 making it suitable for advanced electronic signature
An advanced electronic signature (AdES) is an electronic signature that has met the requirements set forth under EU Regulation No 910/2014 (eIDAS-regulation) on electronic identification and trust services for electronic transactions in the Europe ...
s. This is published by ETSI as TS 102 778.
File attachments
PDF files can have file attachments which processors may access and open or save to a local filesystem.
Metadata
PDF files can contain two types of metadata. The first is the Document Information Dictionary, a set of key/value fields such as author, title, subject, creation and update dates. This is optional and is referenced from Info
key in the trailer of the file. A small set of fields is defined and can be extended with additional text values if required. This method is deprecated in PDF 2.0.
In PDF 1.4, support was added for Metadata Streams, using the Extensible Metadata Platform (XMP) to add XML standards-based extensible metadata as used in other file formats. PDF 2.0 allows metadata to be attached to any object in the document, such as information about embedded illustrations, fonts, images as well as the whole document (attaching to the document catalog), using an extensible schema.
PDF documents can also contain display settings, including the page display layout and zoom level in a Viewer Preferences object. Adobe Reader uses these settings to override the user's default settings when opening the document. The free Adobe Reader cannot remove these settings.
Accessibility
PDF files can be created specifically to be accessible for people with disabilities. PDF file formats in use can include tags, text equivalents, captions, audio descriptions, and more. Some software can automatically produce tagged PDFs, but this feature is not always enabled by default. Leading screen readers, including JAWS, Window-Eyes, Hal, and Kurzweil 1000 and 3000 can read tagged PDF. Moreover, tagged PDFs can be re-flowed and magnified for readers with visual impairments. Adding tags to older PDFs and those that are generated from scanned documents can present some challenges.
One of the significant challenges with PDF accessibility is that PDF documents have three distinct views, which, depending on the document's creation, can be inconsistent with each other. The three views are (i) the physical view, (ii) the tags view, and (iii) the content view. The physical view is displayed and printed (what most people consider a PDF document). The tags view is what screen readers and other assistive technologies use to deliver high-quality navigation and reading experience to users with disabilities. The content view is based on the physical order of objects within the PDF's content stream and may be displayed by software that does not fully support the tags' view, such as the Reflow feature in Adobe's Reader.
PDF/UA, the International Standard for accessible PDF based on ISO 32000-1 was first published as ISO 14289–1 in 2012 and establishes normative language for accessible PDF technology.
Multimedia
''Rich Media PDF'' is a PDF file including interactive content that can be embedded or linked within the file. It can contain images, audio, video content or buttons. For example, if the interactive PDF is a digital catalog for an E-commerce business, products can be listed on the PDF pages, can be added images, links to the website and buttons to order directly from there.
Forms
''Interactive Forms'' is a mechanism to add forms to the PDF file format. PDF currently supports two different methods for integrating data and PDF forms. Both formats today coexist in the PDF specification:
* AcroForms (also known as Acrobat forms), introduced in the PDF 1.2 format specification and included in all later PDF specifications.
* XML Forms Architecture (XFA) forms, introduced in the PDF 1.5 format specification. Adobe XFA Forms are not compatible with AcroForms. XFA was deprecated from PDF with PDF 2.0.
AcroForms were introduced in the PDF 1.2 format. AcroForms permit using objects (''e.g.'' text boxes, Radio buttons, ''etc.'') and some code (''e.g.'' JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
). Alongside the standard PDF action types, interactive forms (AcroForms) support submitting, resetting, and importing data. The "submit" action transmits the names and values of selected interactive form fields to a specified uniform resource locator (URL). Interactive form field names and values may be submitted in any of the following formats, (depending on the settings of the action's ExportFormat, SubmitPDF, and XFDF flags):
; HTML Form format: HTML 4.01 Specification since PDF 1.5; HTML 2.0 since 1.2
; Forms Data Format (FDF): based on PDF, uses the same syntax and has essentially the same file structure, but is much simpler than PDF since the body of an FDF document consists of only one required object. Forms Data Format is defined in the PDF specification (since PDF 1.2). The Forms Data Format can be used when submitting form data to a server, receiving the response, and incorporating it into the interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. FDF was originally defined in 1996 as part of ISO 32000-2:2017.
; XML Forms Data Format (XFDF): (external XML Forms Data Format Specification, Version 2.0; supported since PDF 1.5; it replaced the "XML" form submission format defined in PDF 1.4) the XML version of Forms Data Format, but the XFDF implements only a subset of FDF containing forms and annotations. Some entries in the FDF dictionary do not have XFDF equivalents – such as the Status, Encoding, JavaScript, Page's keys, EmbeddedFDFs, Differences, and Target. In addition, XFDF does not allow the spawning, or addition, of new pages based on the given data; as can be done when using an FDF file. The XFDF specification is referenced (but not included) in PDF 1.5 specification (and in later versions). It is described separately in ''XML Forms Data Format Specification''. The PDF 1.4 specification allowed form submissions in XML format, but this was replaced by submissions in XFDF format in the PDF 1.5 specification. XFDF conforms to the XML standard. XFDF can be used in the same way as FDF; e.g., form data is submitted to a server, modifications are made, then sent back and the new form data is imported in an interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. As of August, 2019, XFDF 3.0 is an ISO/IEC standard under the formal name ''ISO 19444-1:2019 - Document management — XML Forms Data Format — Part 1: Use of ISO 32000-2 (XFDF 3.0)''. This standard is a normative reference of ISO 32000-2.
; PDF
The entire document can be submitted rather than individual fields and values, as was defined in PDF 1.4.
AcroForms can keep form field values in external stand-alone files containing key-value pairs. The external files may use Forms Data Format (FDF) and XML Forms Data Format (XFDF) files. The usage rights (UR) signatures define rights for import form data files in FDF, XFDF and text ( CSV/ TSV) formats, and export form data files in FDF and XFDF formats.
In PDF 1.5, Adobe Systems introduced a proprietary format for forms; Adobe XML Forms Architecture (XFA). Adobe XFA Forms are not compatible with ISO 32000's AcroForms feature, and most PDF processors do not handle XFA content. The XFA specification is referenced from ISO 32000-1/PDF 1.7 as an external proprietary specification, and was entirely deprecated from PDF with ISO 32000-2 (PDF 2.0).
Licensing
Anyone may create applications that can read and write PDF files without having to pay royalties to Adobe Systems; Adobe holds patents to PDF, but licenses them for royalty-free use in developing software complying with its PDF specification.
Security
In November 2019, researchers from Ruhr University Bochum and Hackmanit GmbH published attacks on digitally signed PDFs . They showed how to change the visible content in a signed PDF without invalidating the signature in 21 of 22 desktop PDF viewers and 6 of 8 online validation services by abusing implementation flaws.
At the same conference, they additionally showed how to exfiltrate the plaintext of encrypted content in PDFs. In 2021, they showed new so-called ''shadow attacks'' on PDFs that abuse the flexibility of features provided in the specification. An overview of security issues in PDFs regarding denial of service, information disclosure, data manipulation
Statistics, when used in a misleading fashion, can trick the casual observer into believing something other than what the data shows. That is, a misuse of statistics occurs when a statistical argument asserts a falsehood. In some cases, the misus ...
, and Arbitrary code execution
In computer security, arbitrary code execution (ACE) is an attacker's ability to run any commands or code of the attacker's choice on a target machine or in a target process. An arbitrary code execution vulnerability is a security flaw in softw ...
attacks was presented by Jens Müller.
PDF attachments carrying viruses were first discovered in 2001. The virus, named ''OUTLOOK.PDFWorm'' or ''Peachy'', uses Microsoft Outlook to send itself as an attached Adobe PDF file. It was activated with Adobe Acrobat, but not with Acrobat Reader.
From time to time, new vulnerabilities are discovered in various versions of Adobe Reader, prompting the company to issue security fixes. Other PDF readers are also susceptible. One aggravating factor is that a PDF reader can be configured to start automatically if a web page has an embedded PDF file, providing a vector for attack. If a malicious web page contains an infected PDF file that takes advantage of a vulnerability in the PDF reader, the system may be compromised even if the browser is secure. Some of these vulnerabilities are a result of the PDF standard allowing PDF documents to be scripted with JavaScript. Disabling JavaScript execution in the PDF reader can help mitigate such future exploits, although it does not protect against exploits in other parts of the PDF viewing software. Security experts say that JavaScript is not essential for a PDF reader and that the security benefit that comes from disabling JavaScript outweighs any compatibility issues caused. One way of avoiding PDF file exploits is to have a local or web service convert files to another format before viewing.
On March 30, 2010 security researcher Didier Stevens reported an Adobe Reader and Foxit Reader exploit that runs a malicious executable if the user allows it to launch when asked.
Software
Viewers and editors
PDF viewers are generally provided free of charge, and many versions are available from a variety of sources.
There are many software options for creating PDFs, including the PDF printing capabilities built into macOS
macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...
, iOS
iOS (formerly iPhone OS) is a mobile operating system created and developed by Apple Inc. exclusively for its hardware. It is the operating system that powers many of the company's mobile devices, including the iPhone; the term also include ...
, and most Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...
distributions, LibreOffice
LibreOffice () is a free and open-source office productivity software suite, a project of The Document Foundation (TDF). It was forked in 2010 from OpenOffice.org, an open-sourced version of the earlier StarOffice. The LibreOffice suite co ...
, Microsoft Office 2007
Microsoft Office 2007 (codenamed Office 12) is an office suite for Windows, developed and published by Microsoft. It was officially revealed on March 9, 2006 and was the 12th version of Microsoft Office. It was released to manufacturing on Novem ...
(if updated to SP2) and later, WordPerfect 9, Scribus
Scribus () is free and open-source desktop publishing (DTP) software available for most desktop operating systems. It is designed for layout, typesetting, and preparation of files for professional-quality image-setting equipment. Scribus can a ...
, numerous PDF print drivers for Microsoft Windows, the pdfTeX typesetting system, the DocBook
DocBook is a semantic markup language for technical documentation. It was originally intended for writing technical documents related to computer hardware and software, but it can be used for any other sort of documentation.
As a semantic languag ...
PDF tools, applications developed around Ghostscript
Ghostscript is a suite of software based on an interpreter for Adobe Systems' PostScript and Portable Document Format (PDF) page description languages. Its main purposes are the rasterization or rendering of such page description language file ...
and Adobe Acrobat itself as well as Adobe InDesign, Adobe FrameMaker, Adobe Illustrator
Adobe Illustrator is a vector graphics editor and design program developed and marketed by Adobe Inc. Originally designed for the Apple Macintosh, development of Adobe Illustrator began in 1985. Along with Creative Cloud (Adobe's shift to month ...
, Adobe Photoshop. Google
Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
's online office suite Google Docs allows for uploading and saving to PDF. Some web apps offer free PDF editing and annotation tools.
The Free Software Foundation once thought of as one of their high priority projects to be "developing a free, high-quality and fully functional set of libraries and programs that implement the PDF file format and associated technologies to the ISO 32000 standard." In 2011, however, the GNU PDF project was removed from the list of "high priority projects" due to the maturation of the Poppler library, which has enjoyed wider use in applications such as Evince with the GNOME desktop environment. Poppler is based on Xpdf code base. There are also commercial development libraries available as listed in List of PDF software
This is a list of links to articles on software used to manage Portable Document Format (PDF) documents. The distinction between the various functions is not entirely clear-cut; for example, some viewers allow adding of annotations, signatures, e ...
.
The Apache PDFBox project of the Apache Software Foundation is an open source Java library for working with PDF documents. PDFBox is licensed under the Apache License.
Printing
Raster image processors (RIPs) are used to convert PDF files into a raster format suitable for imaging onto paper and other media in printers, digital production presses and prepress Prepress is the term used in the Printing and Publishing industries for the processes and procedures that occur between the creation of a print layout and the final printing. The prepress process includes the preparation of artwork for press, media ...
in a process known as rasterisation
In computer graphics, rasterisation (British English) or rasterization (American English) is the task of taking an image described in a vector graphics format (shapes) and converting it into a raster image (a series of pixels, dots or lines, whi ...
. RIPs capable of processing PDF directly include the Adobe PDF Print Engine from Adobe Systems and Jaws and the Harlequin RIP from Global Graphics.
In 1993, the Jaws raster image processor from Global Graphics became the first shipping prepress RIP that interpreted PDF natively without conversion to another format. The company released an upgrade to their Harlequin RIP with the same capability in 1997.
Agfa-Gevaert
Agfa-Gevaert N.V. (Agfa) is a Belgian-German multinational corporation that develops, manufactures, and distributes analogue and digital imaging products, software, and systems. It has three divisions:
* Agfa Graphics, which offers integrated pr ...
introduced and shipped Apogee, the first prepress workflow system based on PDF, in 1997.
Many commercial offset printers have accepted the submission of press-ready PDF files as a print source, specifically the PDF/X-1a subset and variations of the same. The submission of press-ready PDF files is a replacement for the problematic need for receiving collected native working files.
In 2006, PDF was widely accepted as the standard print job format at the Open Source Development Labs Printing Summit. It is supported as a print job format by the Common Unix Printing System and desktop application projects such as GNOME, KDE, Firefox
Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements current ...
, Thunderbird
Thunderbird, thunder bird or thunderbirds may refer to:
* Thunderbird (mythology), a legendary creature in certain North American indigenous peoples' history and culture
* Ford Thunderbird, a car
Birds
* Dromornithidae, extinct flightless birds ...
, LibreOffice
LibreOffice () is a free and open-source office productivity software suite, a project of The Document Foundation (TDF). It was forked in 2010 from OpenOffice.org, an open-sourced version of the earlier StarOffice. The LibreOffice suite co ...
and OpenOffice OpenOffice or open office may refer to:
Computing Software
* OpenOffice.org (OOo), a discontinued open-source office software suite, originally based on StarOffice
* Apache OpenOffice (AOO), a derivative of OOo by the Apache Software Foundation, w ...
have switched to emit print jobs in PDF.
Some desktop printers also support direct PDF printing, which can interpret PDF data without external help.
Native display model
PDF was selected as the "native" metafile format for Mac OS X, replacing the PICT
The Picts were a group of peoples who lived in what is now northern and eastern Scotland (north of the Firth of Forth) during Late Antiquity and the Early Middle Ages. Where they lived and what their culture was like can be inferred from ear ...
format of the earlier classic Mac OS. The imaging model of the Quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
graphics layer is based on the model common to Display PostScript
Display PostScript (or DPS) is a 2D graphics engine system for computers which uses the PostScript (PS) imaging model and language (originally developed for computer printing) to generate on-screen graphics. To the basic PS system, DPS adds a num ...
and PDF, leading to the nickname ''Display PDF''. The Preview application can display PDF files, as can version 2.0 and later of the Safari web browser. System-level support for PDF allows Mac OS X applications to create PDF documents automatically, provided they support the OS-standard printing architecture. The files are then exported in PDF 1.3 format according to the file header. When taking a screenshot under Mac OS X versions 10.0 through 10.3, the image was also captured as a PDF; later versions save screen captures as a PNG file, though this behavior can be set back to PDF if desired.
Annotation
Adobe Acrobat is one example of proprietary software that allows the user to annotate, highlight, and add notes to already created PDF files. One UNIX application available as free software (under the GNU General Public License
The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the four freedoms to run, study, share, and modify the software. The license was the first copyleft for general ...
) is PDFedit
PDFedit is a free PDF editor for Unix-like operating systems (including Cygwin on top of Windows). It does not support editing protected or encrypted PDF files or word processor-style text manipulation, however.freeware Foxit Reader
Foxit PDF Reader (formerly Foxit Reader) is a multilingual freemium PDF (Portable Document Format) tool that can create, view, edit, digitally sign, and print PDF files. Foxit Reader is developed by Fuzhou, China-based Foxit Software. Early ver ...
, available for Microsoft Windows, macOS
macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...
and Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...
, allows annotating documents. Tracker Software's PDF-XChange Viewer
PDF-XChange Viewer is a freemium PDF reader for Microsoft Windows. It was available in a free and a paid version. Its further development has been stopped in favour of the freemium PDF-XChange Editor. It supports saving PDF forms (AcroForms), ...
allows annotations and markups without restrictions in its freeware alternative. Apple's macOS
macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...
's integrated PDF viewer, Preview, does also enable annotations as does the open-source software Skim, with the latter supporting interaction with LaTeX, SyncTeX, and PDFSync and integration with BibDesk reference management software. Freeware Qiqqa
Qiqqa (pronounced "Quicker") is a free and opensource software that allows researchers to work with thousands of PDFs. It combines PDF reference management tools, a citation manager, and a mind map brainstorming tool. It integrates with Microsoft ...
can create an annotation report that summarizes all the annotations and notes one has made across their library of PDFs. The Text Verification Tool exports differences in documents as annotations and markups.
There are also web annotation systems that support annotation in pdf and other documents formats. In cases where PDFs are expected to have all of the functionality of paper documents, ink annotation is required.
Alternatives
The Open XML Paper Specification is a competing format used both as a page description language and as the native print spooler format for Microsoft Windows since Windows Vista.
Mixed Object: Document Content Architecture is a competing format. MO:DCA-P is a part of Advanced Function Presentation.
See also
* Web document
* XSL Formatting Objects
XSL-FO (XSL Formatting Objects) is a markup language for XML document formatting that is most often used to generate PDF files. XSL-FO is part of Extensible Stylesheet Language, XSL (Extensible Stylesheet Language), a set of W3C technologies design ...
References
Further reading
*
* PDF 2.0
* PDF 2.0
PDF 1.7 (ISO 32000-1:2008)
PDF 1.7
an
errata to 1.7
PDF 1.6
() an
errata to 1.6
PDF 1.5
an
errata to 1.5
PDF 1.4
() an
errata to 1.4
PDF 1.3
() an
errata to 1.3
External links
PDF Association
– The PDF Association is the industry association for software developers producing or processing PDF files.
Adobe: PostScript vs. PDF
– Official introductory comparison of PS, EPS vs. PDF.
* – Information about PDF/E and PDF/UA specification for accessible documents file format (archived by The Wayback Machine)
PDF/A-1 ISO standard
published by the International Organization for Standardization
The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Art ...
(with corrigenda)
PDF Reference and Adobe Extensions to the PDF Specification
– Introduction to PDF vs. PostScript and PDF internals (up to v1.3)
The Camelot Paper
– the paper in which John Warnock outlined the project that created PDF
Everything you wanted to know about PDF but was afraid to ask
�
recording of a talk by Leonard Rosenthol
(45 mins) ( Adobe Systems) at TUG 2007
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Computer-related introductions in 1993
Adobe Inc.
Digital press
Electronic documents
Graphics file formats
ISO standards
Office document file formats
Open formats
Page description languages
Vector graphics