Go to Start / Default programs / Associate a file or protocol with a program and scroll to .pdf.
It should indicate Adobe Reader as the default program but it probably doesn't. Click on change and then see if Adobe Reader is one of the listed options. If so, choose it and save your work. You will now be able to open PDF files.
If Adobe Reader is not one of the options, click on browse and locate the Adobe Reader executable file (it will probably be somewhere in a sub-directory of the Program Files sub-directory) and then select that as the default program. Again, once done you should once again be able to open PDF files.
Chances are good that the Microsoft Word Viewer set itself up as the default for PDF files and now there's no place for them to go (they have no valid association). The reason you have no options to open with is that there are probably no programs setup as associations for PDF files. You could also try uninstalling Adobe Reader and then re-installing it.
That should re-establish the association and fix the problem. How you go about fixing the problem is up to you.