In reading through other forums it seems that this is a common bug with Adobe Reader X. We are a medical company and have a lot of CCR and CR's that are in PDF form. Seems that whenever one of these email attachments is involved where the C is next to the R in the PDF file name it throws up the following error:
"Before proceeding you must first launch Adobe Acrobat and accept the End User License Agreement" - except we already accepted the license agreement on the first launch of Adobe Reader.
The workaround that was suggested is to always enable protected mode but when we enable protected mode we can't open the PDF that way either. "There was an error opening the document. Access Denied."
So the only OTHER workaround I've found is to open Adobe Reader first before trying to open the attachments and then it works. But my user's of course like things simple and would like to be able to just open the attachment without opening the Adobe Reader beforehand.