PDF is the Acronym for Portable Document Format.
Documents saved in this format can be opened by a version of Adobe Acrobat or Adobe Acrobat Reader. If you do not have Adobe Acrobat Reader in your computer it is available for free on the internet and is easy to download and install.
It is a wonderful way to share documents because anyone can get a free copy of the program without having to purchase other programs to view a variety of formats.
I think it was developed for the US Government as a method of sharing information with everyon so that they did not need to purchase something like Microsoft Mord.