UMD - what do you usually use to view .pdf files? Adobe Acrobat, or Adobe Reader?
also, from what I read, this could be an issue with having multiple .pdf readers installed, if you have both Acrobat AND Reader installed on the same machine.
If you only have Reader, re-install it. when installed, it detects what Browsers you have and downloads the plugins for it. I don't know where to get those plugins, because i never needed to get them. I read .pdfs in Google Docs Viewer, most of the time. but I don't recall having many issues with Adobe Reader either.