“PDF” stands for ‘portable document format’ and is a popular format for downloadable documents, forms and user guides and manuals.
For years, Adobe Reader was the only way to view these files, and while it is still available, it has become a large, unwieldy program (some call it ‘bloatware’) that can slow your system down. Typically, most PC builders preinstall Adobe Reader on their computers, but you can remove it, and I suggest you do.
Corrine also provides a link to several free, open source alternatives to Adobe Reader.
Presently, I am running SumatraPDF on Windows.
Please have a look at Corrine’s post for more information.