'PDF' is an acronym for 'Portable Document Format'. A PDF file contains one or more page images, each of which you can zoom in on or out from.
PDF files are especially useful when you want to preserve the original appearance and format of a document, especially when sharing working between different computers/operating systems. The The PDF file is designed to look the same on a screen and in print, regardless of what kind of computer or printer someone is using and regardless of what software package was originally used to create it. PDF files are also highly compressed, so that complex information can be downloaded efficiently.
Read more on generating and work with PDF documents.