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Portable Document Format (PDF)

PDF is an acronym for Portable Document Format. It is a multi – platform supported file format developed and owned by Adobe Systems (initiated by the co – founder John Warnock under the project Camelot). The peculiarity of the PDF file format is that it can be used to read documents independent of the basic computing resources such as the hardware being used, the running application software as well as the Operating System being used. PDF file has the caliber to condense text, font, and graphics in a single document.

The PDF file belongs to the category of document format.

PDF file is independent of the hardware, software or Operating System it is being traversed on. The basic software required for viewing PDF files is Adobe Acrobat Reader.

Note: - Adobe Systems are patent to PDF but the license structure is royalty free. In simple words, any individual or organization can create applications that can be used to develop or read PDF files without paying any capital to the Adobe Systems.

The PDF file is initially owned and developed by Adobe Systems.

  • application/pdf
  • application/x-pdf
  • application/acrobat
  • applications/vnd.pdf
  • text/pdf
  • text/x-pdf

PDF files use digital signatures for authentication.

The structure of PDF files is based on COS format and PostScript Imaging model.


COS is an acronym for "Carousel" Object Structure. The COS file format has a tree based structure that is comprised of eight peculiar object types: -

  • Boolean values: – true or false
  • Numbers
  • Strings
  • Names
  • Arrays (Linear Data Structure)
  • Dictionaries (an indexed library of Names)
  • Streams (huge data repositories)
  • The null object (empty)

Based on this COS model; PDF files can be categorized into two layouts: -

  • Non – linear
  • Linear

Imaging Model – PostScript

The imaging model being used by the PDF format; up to very much extent, resembles the PostScript. PDF graphics makes use of a device independent Cartesian coordinate system that defines the surface of a page.

The most frequent errors that occur in PDF file format are of accessibility type.

PDF Not Tagged

The PDF documents, if not tagged, generate an absurd error. This makes it difficult for the applications including Adobe Systems to recognize all the available sections of the document.

PDF Not Having A Title

PDF documents without title result in error and are not 100% accessible.

PDF Document Language Not Defined

For the applications to render the data of PDF file in an acceptable format, the most important criteria are to define the language of the PDF file. Failing to do so may result in an error.

Image-Only PDFs

For the sake of ease of screen readers, the PDF files must contain texts in addition to the images. Reason being, the screen readers may not find it compatible if the PDF documents are only made of images.

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