If you are not able to open PDF files, then you probably need to re-associate your PDF files with Adobe Reader, or Adobe Pro if you have that version.
Right click on any PDF file (on your desktop or anywhere on a local, not network or internet file) You are just going to be telling your computer what to do when you double click on a PDF, so any file that is supposed to be PDF that is on your local computer is fine. It needs to be a local file, not a network or Internet file.
Select OPEN WITH->
Select Choose Default Program (bottom of the list)
Select Adobe Reader or(Adobe Pro if you have that).
This applies to Windows computers only. Macbooks will use the Mac Preview option.