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Portable Document Format or PDF is a file format developed by Adobe Systems for representing documents in a manner that is independent of the original application software, hardware, and operating system used to create those documents. A PDF file can describe documents containing any combination of text, graphics, and images in a device independent and resolution independent format. These documents can be one page or thousands of pages, very simple or extremely complex with a rich use of fonts, graphics, color, and images. PDF is an open standard, and anyone may write applications that can read or write PDFs royalty-free.
In addition to encapsulating text and graphics, PDF files are most appropriate for encoding the exact look of a document in a device-independent way. In contrast, markup languages such as HTML defer many display decisions to a rendering device such as a browser, and will not look the same on different computers.
Free readers for many platforms are available:
- Adobe Reader by Adobe Systems
- Xpdf for POSIX-like systems with the X Window System
- KPDF, a viewer based on Xpdf for KDE
- GPdf, a derivative of Xpdf for GNOME
- Evince, a document viewer for GNOME (fork of GPdf)
Proper subsets of PDF, collectively called PDF/X, have been standardized by ISO.
PDF is primarily the combination of three technologies:
- a cut-down form of PostScript for generating the layout and graphics,
- a font-embedding/replacement system to allow fonts to travel with the documents, and
- a structured storage system to bundle these elements into a single file, with data compression where appropriate.
PostScript is a page description language that is run in an interpreter to generate an image. This process requires a fair amount of resources.
PDF is a subset of those PostScript language elements that define the graphics, and only requires a very simple interpreter. For instance, flow control commands like
loop are removed, while graphics commands such as
That means that the process of turning PDF back into a graphic is a matter of simply reading the description, rather than running a program in the PostScript interpreter. However, the entire PostScript world in terms of fonts, layout and measurement remains intact.
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 the document references are also collected; and finally everything is compressed into a single file.
As a document format, PDF has several advantages over PostScript:
- A PDF document resides in a single file, whereas the same document in PostScript may span multiple files (graphics, etc.) and probably occupies more space.
- PDF contains already-interpreted results of the PostScript source code, so it is less computation-intensive and faster to open, and there is a more direct correspondence between changes to items in the PDF page description and changes to the resulting appearance of the page.
- PDF (starting from version 1.4) supports true object transparency while PostScript does not.
- PDF allows font embedding to ensure that the "proper" fonts are displayed. While this is possible with PostScript, such files cannot normally be distributed freely because of font licensing agreements.
PDF and accessibility
PDF can be accessible to people with disabilities. Current PDF file formats can include tags (essentially XML), text equivalents, captions and audio descriptions, and other accessibility features. Some software can output tagged PDFs automatically. Tagged PDFs can be reflowed and zoomed for low-vision readers.
However, many problems remain, not least of which is the difficulty in adding tags to existing or "legacy" PDFs; for example, if PDFs are generated from scanned documents, accessibility tags and reflowing are unavailable and must be created either by hand or using OCR techniques. Also, these processes themselves are often inacessible to the people who would benefit from them. Nonetheless, well-made PDFs can be a valid choice as long-term accessible documents. (Work is being done on a PDF variant based on PDF 1.4. The PDF/A or PDF-Archive is specifically scaled down for archival purposes.)
PDF on the Web
Because HTML/XHTML rendering across web browsers has historically been inconsistent and sometimes unpredictable, PDF use online is becoming increasingly common. This is particularly true for order forms, catalogues, brochures, and other documents which are primarily formatted for printing. The ubiquity of the Adobe Reader web browser plugin, however, has inspired some (mostly corporate) web authors to publish a wider variety of information as PDF. This trend is compounded by the simple operation and wide corporate availability of WYSIWYG PDF authoring tools. While the end user experience of an XHTML document can vary significantly depending on browser, platform, and screen resolution, a PDF file can be reasonably expected to look exactly the same to every viewer.
Critics of this practice cite several reasons for avoiding it. The major one is that the inflexibility of PDF rendering makes it difficult to read on screen: it does not adapt to the window size nor the reader's preferred font size and font family, as classic XHTML web page does. Accessibility, particularly by the blind or sight-impaired is a common issue . PDF files tend to be significantly larger than XHTML/SVG files presenting the same information, making it difficult or impossible for users with low-bandwidth connections to view them. Adobe Acrobat Reader, the de facto standard PDF viewer, has historically been slow to start and caused browser instability, particularly when run alongside other browser plugins (though the release of Adobe Reader 7 addressed many of these concerns). Acrobat reader is also unavailable in current versions on many alternative operating systems and is distributed under a proprietary license unacceptable to some users.
Currently, only one major web browser, Safari, natively supports PDF. Users of other clients must run a separate application to access these documents online.
- Acrobat, for creating PDFs
- Adobe Reader, for viewing them
- Create Adobe PDF Online — Online service for creating PDF files from many different document types, including Microsoft Word
- Online conversion tools for Adobe PDF documents — The official Adobe online tool for converting from PDF to Text or HTML; also accepts emailed documents
- PDF Specification, also available as a book describing PDF 1.4 (ISBN 0201758393)
- Adobe: PostScript vs. PDF
- History of PDF at prepressure.com
- The Camelot Paper — the paper in which John Warnock outlined the project that created PDF
- Planet PDF — Extensive archive of articles about using and developing for the PDF format.
- PDF Zone has a number of PDF utilities including alternative writer software.