Activex association problem: pdfs in viewer pane

Reinstalling Adobe Reader will probably fix it, so give that a try.

If it's still broken after that, see if other programs are able to view the file. e.g. If you double-click it or drop it on IE, does it show up?

If you're on 64-bit Windows, make sure the PDF-ActiveX plugin is disabled and make sure you're using the latest ActiveX+Preview+Office+Web plugin. (You can do that on 32-bit Windows as well, but it isn't vital like it is on 64-bit.)