PDF stands for Portable Document Format. 
Portable Document Format (PDF) is a file format developed by Adobe Systems for representing documents on screen in a consistent manner regardless of the software used to create the document, or the computer operating system.
These documents can be one page or thousands of pages, very simple or extremely complex, with a rich use of fonts, graphics, colour, and images.
PDF is an open standard, and anyone may write applications that can read or write PDFs royalty-free.
Free readers for many platforms are available for download from the Adobe website http://www.adobe.com/products/acrobat/readstep2.html
There are also several free open source readers, including
Muhammad Irfan 