Well this one derailed me for a bit...
We had IIS setup with automatic, immediate content expiration for a virtual server, and I had written a chunk of ASP.NET that would stream a file through a Web Service (don't ask why, there's good reasons) down to the client. When those files were PDFs, that worked file. One of my other team members write similar code to download a PDF from Reporting Services to the end-client. His would fail on machines running Acrobat Reader, but would work on machines running Acrobat Professional (6.0.x versions.)
The culprit was letting IIS add an automatic and immediate expiration header. When I removed that, all versions started working.
Grant, it should have worked or not for either of us -- but I did notice that I used a pop-up window to stream the file down where my cohort just opened it in the current frame. Wouldn't expect that to make a difference.
I recall having this issue with "Expire Immediately" and MS Office documents in the past, so it was one of the first things I looked at after making sure the code at least looked correct.