PDF Files

Portable Document Format; also known as PDF is a computer document which look exactly like the original documents the person/s have created. The document can be shared with others who are able to open & see/read the document (regardless of which software they use). PDF files were developed by Adobe Systems, and served as a way for people using multiple word processing systems to share finished documents. An advantage of the PDF file is that the file can be locked by the creator of the document and can't be edited by others, and that's even is they also have the software! Because the documents are secure, the originator of the document maintains a high level of control, even when the document has been distributed widely.

Reference/s: http://www.wisegeek.com/what-are-pdf-files.htm