Portable Document Format (PDF) is a file format developed by Adobe Systems for representing two dimensional documents in a device independent and resolution independent format. Each PDF file encapsulates a complete description of a 2D document that includes the text, fonts, images, and 2D vector graphics that compose the document. Importantly, PDF files don't encode information that is specific to the application software, hardware, or operating system used to create or view the document. This feature ensures that a valid PDF will render exactly the same regardless of its origin or destination. PDF is also an open standard in the sense that anyone may create applications that read and write PDF files without having to pay royalties to Adobe Systems.
PDF files are most appropriately used to encode the exact look of a document in a device-independent way. While the PDF format can describe very simple one page documents, it may also be used for many page, complex documents that use a smorgasbord of different fonts, graphics, colors, and images.
Types of content
A PDF file of a map, for example, is often a combination of vector graphics layer, text, and raster graphics. A general reference map of the US  uses:
* text stored as such — scalable, and also one can copy the text
* vector graphics for coastlines, lakes, rivers, highways, markings of cities, and Interstate highway symbols — on zooming in, the curves remain sharp, they do not appear as consisting of enlarged pixels (i.e. rectangles of pixels)
* raster graphics for showing mountain relief — on zooming in, this consists of enlarged pixels
To appreciate the difference between vector and raster graphics, compare the general reference map to the CIA World Factbook's European map. If you zoom in on the former, you'll see that the (vector) coastline is nicely fitted to the (raster) mountain relief. If you zoom in on the latter, in contrast, the blue of the sea is not nicely fitted to the vector graphics coast line, and the land-sea boundary is much cruder.
Some PDFs have no raster graphics at all. For example, see the Factbook's map of the Arctic.
Tools exist, such as pdfimages (bundled with Xpdf) to extract the raster images from a PDF file. This can be extremely useful if the PDF is simply a collection of scanned pages.
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, such as Adobe InDesign, can output tagged PDFs automatically. Leading screen readers, including Jaws, Window-Eyes, and Hal, can read tagged PDFs; current versions of the Acrobat and Acrobat Reader programs can also read PDFs out loud. Moreover, 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.)
Microsoft Word documents can be converted into accessible PDFs, but only if the Word document is written with accessibility in mind - for example, using styles, correct paragraph mark-up and "alt" (alternative) text for images, and so on.
source from - www.wikipedia.org
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