What are PDF documents?
Portable Document Format (PDF) is a file format developed by Adobe to deliver and render electronic documents created for print. They are viewed using a helper agent which displays the document on the screen making them independent of operating system, authoring software and display device. Since this format is based on PostScript page description language used for printing, it preserves the exact look (style, layout, formatting, fonts, images, etc.) of the original document and is usually created by "painting" the source document to an eelctronic file. Thus the original PDF file formats were a "printed" electronic image of the source file.
PDF Documents Types:
- Image (unstructured) or a graphical representation of the original document.
Since they are images, they are inaccessible to assistive technology like screen readers.
- Searchable Image (structured) consisting of a combination
of images (vector and raster) plus text from the original document. The
text is searchable and thus is partially accessible to screen reader. These
documents maintain the logical structure of the original document.
- Tagged Document which is a true electronic document with
semantic markup, presentation attributes, searchable text and information
on non textual elements.
- Similar to other mark-up languages, tagged PDF files have a hierarchy of “tags” encoding the semantic structure. The position of the tag within the tree represents the reading order of the document. When well crafted, these documents are accessible to assistive technology like screen readers. At this time only Acrobat Adobe is capable of creating a tagged PDF document.
How are PDF file formats created?
Any electronic document can be converted into a PDF file. While Adobe’s Acrobat and Reader are the most popular tools used to generate and view PDF formats, there are many other serverside and desktops tools available that will generate and view PDF files formats from any source document created by any application. PDF format is also a native print format built into Mac OSX.
PDF documents are generally created:
- As a scanned image of a print document saved as PDF file (structured)
- From PostScript files or other publishing tools using Acrobat Distiller (structured)
- Directly "printed" from word processing, TeX or other publishing tools using other desktop and serverside PDF generators (structured /unstructured)
- "Printed" as a PDF file from any application using Mac OSX (unstructured)
- Generated from recent versions of MS Office and Adobe publishing tools (tagged)
