Portable Document Format
Portable Document Format (PDF) | |
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File extension: | .pdf |
MIME type: | application/pdf |
Type code: | 'PDF ' (including a single space) |
Uniform Type Identifier: | com.adobe.pdf |
Magic: | %PDF |
Developed by: | Adobe Systems |
The Portable Document Format (PDF) is the file format created by Adobe Systems in 1993 for document exchange. PDF is used for representing two-dimensional documents in a device-independent and display resolution-independent fixed-layout document format. Each PDF file encapsulates a complete description of a 2-D document (and, with Acrobat 3-D, embedded 3-D documents) that includes the text, fonts, images, and 2-D vector graphics that compose the document.
PDF is an open standard, and is now being prepared for submission as an ISO standard.[1]
Contents |
History
When the PDF first came out in the early 1990s, its general adoption was slow.[2] At that time, the PDF-creation tools (Acrobat) and the viewing and printing software had to be bought. Early versions of PDF had no support for external hyperlinks, reducing its usefulness on the World Wide Web; the additional size of the PDF document compared to plain text meant significantly longer download times over the slower modems common at the time, and rendering the files was slow on less powerful machines. Additionally, there were competing formats such as Envoy, and even Adobe's own PostScript format (.ps); in those early years, the PDF file was mainly popular in desktop publishing workflow.
Adobe soon started free distribution of the Acrobat Reader (now Adobe Reader) program, and continued supporting the original PDF, which eventually became the de facto standard for printable documents.
The PDF file format has changed several times, as new versions of Adobe Acrobat have been released. There have been eight versions of PDF: 1.0 (1993), 1.1 (1994), 1.2 (1996), 1.3 (1999), 1.4 (2001), 1.5 (2003), 1.6 (2005), and 1.7 (2006), corresponding to Acrobat releases 1.0 to 8.0.
Technology
Anyone may create applications that 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.[3]
The PDF combines three technologies:
- A sub-set of the PostScript page description programming language, for generating the layout and graphics.
- A font-embedding/replacement system to allow fonts to travel with the documents.
- A structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate.
PostScript
PostScript is a page description language run in an interpreter to generate an image, a process requiring many resources. PDF is a file format, not a programming language, i.e. flow control commands such as if
and loop
are removed, while graphics commands such as lineto
remain.
Often, 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 tokenized; any files, graphics, or fonts to which the document refers also are collected; then, everything is compressed to a single file. Therefore, the entire PostScript world (fonts, layout, measurements) remains intact.
As a document format, PDF has several advantages over PostScript:
- PDF contains already tokenized and interpreted results of the PostScript source code, for direct correspondence between changes to items in the PDF page description and changes to the resulting page appearance.
- PDF (from version 1.4) supports true graphic transparency, PostScript does not.
- PostScript is an imperative programming language (with an implicit global state), so instructions accompanying the description of one page can affect the appearance of any following page. Therefore, all preceding pages must be processed in order to determine the correct appearance of a given page; each page in a PDF document is unaffected by the others.
Accessibility
PDF files can be created specifically to be accessible for disabled people. Current PDF file formats can include tags (XML), text equivalents, captions, audio descriptions, et cetera. Some software, such as Adobe InDesign, can automatically produce tagged PDFs. Leading screen readers, including JAWS, Window-Eyes, Hal, and Kurzweil 1000 and 3000 can read tagged PDFs; current versions of the Acrobat and Acrobat Reader programs can also read PDFs aloud. Moreover, tagged PDFs can be re-flowed and magnified for readers with visual impairments. Problems remain with adding tags to older PDFs and those that are generated from scanned documents. In these cases, accessibility tags and re-flowing are unavailable, and must be created either manually or with OCR techniques. These processes are inaccessible to some disabled people. PDF/UA, the PDF/Universal Accessibility Committee, an activity of AIIM, is working on a specification for PDF accessibility based on the PDF 1.6 specification.
One of the major problems 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 read (useful for people with poor eyesight). The content view is displayed when the document is re-flowed to Acrobat (useful for people with mobility disability). For a PDF document to be accessible, the three views must be consistent with each other.
Security
In 2001, PDF format attachments carrying viruses were first discovered. Virus researchers found that the PDF file viruses activated with Adobe Acrobat, but not with Acrobat Reader. As with all file formats, caution is advised. An up-to-date antivirus program is paramount.
Usage restrictions and monitoring
PDFs may be encrypted so that a password is needed to view or edit the contents. The PDF Reference defines both 40-bit and 128-bit encryption, both making use of a complex system of RC4 and MD5. The PDF Reference also defines ways in which third parties can define their own encryption systems for use in PDF.
PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing or printing. The restrictions on copying, editing, or printing depend on the reader software to obey them, so the security they provide is limited. Printable documents especially might be saved instead as bitmaps and subject to OCR.
The PDF Reference has technical details or see [1] for an end-user overview. Like HTML files, PDF files may submit information to a web server. This could be used to track the IP address of the client PC, a process known as phoning home.
Through their product, Adobe provides a method to set security policies on specific documents. This can include requiring a user to authenticate and limiting the time frame a document can be accessed or amount of time a document can be opened while offline. Once a PDF document is tied to a policy server and a specific policy, that policy can be changed or revoked by the owner. This controls documents that are otherwise "in the wild." Each document open and close event can also be tracked by the policy server. Policy servers can be set up privately or Adobe offers a public service through Adobe Online Services.
Subsets
Proper subsets of PDF have been, or are being, standardized under ISO for several constituencies:
- PDF/X for the printing and graphic arts as ISO 15930 (working in ISO TC130)
- PDF/A for archiving in corporate/government/library/etc environments as ISO 19005 (work done in ISO TC171)
- PDF/E for exchange of engineering drawings (work done in ISO TC171)
- PDF/UA for universally accessible PDF files
A PDF/H variant (PDF for Healthcare) is being developed.[4] However, it may consist more in a set of "best practices" than in a specific format or subset.
Mars
- See also: Page description markup language
According to a 7 December 2006 Government Computer News blog, Joab Jackson writes that Adobe is exploring an XML-based next-generation PDF codenamed Mars: http://www.gcn.com/blogs/tech/42740.html
Adobe has published information about the Mars file format at http://www.adobe.com/go/mars and at http://labs.adobe.com/wiki/index.php/Mars.
The format of graphic elements of Mars is sometimes described simply as “SVG”, but according to the 0.8.0 draft specification (§7.4, §7.5) the format is actually merely similar to SVG: it contains both additions to and subtractions from SVG, so it is in general neither viewable by nor creatable with standard SVG tools: some things will look noticeably different between SVG viewers and Mars viewers.
Content
A PDF file is often a combination of vector graphics, text, and raster graphics. The basic types of content in a PDF are:
- text stored as such
- vector graphics for illustrations and designs that consist of shapes and lines
- raster graphics for photographs and other types of image
In later PDF revisions, a PDF document can also support links (inside document or web page), forms, JavaScript (initially available as plugin for Acrobat 3.0), or any other types of embedded contents that can be handled using plug-ins.
PDF 1.6 supports interactive 3D documents embedded in the PDF.
Two PDF files which look similar on a computer screen may be of very different sizes. For example, a high resolution raster image takes more space than a low resolution one. Typically higher resolution is needed for printing documents than for displaying them on screen. Other things that may increase the size of a file is embedding full fonts, especially for Asiatic scripts, and storing text as graphics.
Base 14 Fonts
There are fourteen typefaces that have a special significance to PDF documents: Times Roman (in standard, italic, bold, and bold oblique), Courier (in standard, oblique, bold and bold oblique), Helvetica (in standard, oblique, bold and bold oblique), Symbol and Zapf Dingbats. These should always be present (actually present or a close substitute) and so need not be embedded in a PDF. [2] PDF viewers must know about the metrics of these fonts. Other fonts may be substituted if they are not embedded in a PDF.
Versions
PDF Version | Year of Publication | new features | supported by Adobe Reader version |
---|---|---|---|
1.2 | FlateDecode | Acrobat Reader 3.0 | |
1.3 | 2000 | Acrobat Reader 4.0 | |
1.4 | 2001 | JBIG2 | Acrobat Reader 5.0 |
1.5 | 2003 | JPEG2000 | Adobe Reader 6.0 |
1.6 | 2004 | Adobe Reader 7.0 | |
1.7 | 2007 | Adobe Reader 8.0 |
Implementations
Readers for many platforms are available, such as Adobe Reader, Foxit, Preview, Xpdf, Evince, Okular, and KPDF; there are also front-ends for many platforms to Ghostscript. PDF readers are generally free. There are many software options for creating PDFs, including the PDF printing capability built in to Mac OS X, the multi-platform OpenOffice.org, Microsoft Office 2007 (an additional free download from Microsoft is required), Wordperfect since version 9, numerous PDF print drivers for Microsoft Windows, and Adobe Acrobat itself. There is also specialized software for editing PDF files.
AGFA introduced and shipped Apogee, the very first prepress workflow system based on PDF in 1997.
PDF was selected as the "native" metafile format for Mac OS X, replacing the PICT format of the earlier Mac OS. The imaging model of the Quartz graphics layer of Mac OS X is based on the model common to Display PostScript and PDF, leading to the nickname "Display PDF". The Preview application can display PDF files, and the version of Safari in Mac OS X v10.4 can display PDF files as well. System-level support for PDF allows Mac OS X applications to create PDF documents automatically, provided they support the Print command. When taking a screenshot under Mac OS X versions 10.0 through 10.3, the image was also captured as a PDF; in 10.4 the default behaviour is set to capture as a PNG file, though this behaviour can be set back to PDF if required.
Some printers also support direct PDF printing, which can interpret PDF data without external help. Currently, all PDF capable printers also support PostScript, but most PostScript printers do not support direct PDF printing.
See also
References
- ^ Adobe Systems Inc (29 January 2007). Adobe to Release PDF for Industry Standardization. Retrieved on 2007-01-31.
- ^ Laurens Leurs. The history of PDF. Retrieved on 2007-09-19.
- ^ http://partners.adobe.com/public/developer/support/topic_legal_notices.html
- ^ AIIM (2006-10-20). New Best Practices Guide Addresses Exchange of Healthcare Information. Retrieved on 2007-03-09.
External links
- Download a freeware PDF Reader from Adobe
- PDF Specifications, including the PDF Reference for PDF 1.7, PDF 1.6 (ISBN 0-321-30474-8), PDF 1.5, PDF 1.4 (ISBN 0-201-75839-3), PDF 1.3 (ISBN 0-201-61588-6)
- White Paper: PDF Primer PDF (167 KiB) — A white paper from PDF Tools AG with an introduction into what PDF is and its strengths and weaknesses.
- Adobe: PostScript vs. PDF
- Planet PDF: The PDF User Community
- History of PDF at prepressure.com
- The Camelot Paper — the paper in which John Warnock outlined the project that created PDF
- AIIM — Information about PDF/E specification for engineering
- AIIM — Information about PDF/UA specification for accessible documents
- Mars — Further information about the Adobe Mars Format
- Adobe PDF 101: Quick overview of PDF file format
- Intro to PDF — AGFA booklet: Intro to PDF and PDF/X
- White Paper: Comparing PDF and XPS
- PDF Files Resources
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