We have the exact same issue, but with a different setup.
Internal users do not have issues opening PDF files that are stored on the SharePoint server. The issue is there when users access the same website from the internet, and then try to open the PDF file.
To make the same website accessible from the internet we use Microsoft Forefront UAG. Doing a Fiddler trace we see a HTTP 500 error returned when the action is done to open a PDF file. This HTTP 500 message seems to come from the SP server. Not from the UAG server.
Tried other Adobe Reader versions, and like mentioned on the first page of this topic the older versions do not have problems. Also, using the 64 bit version of the latest Adobe Reader versions seem to work out fine. But we dont want this workaround to be used.
I hope to get more informantion from here soon.