I have a web site hosted by Yahoo!Geocities. For some time I have noticed that pdf files mounted on the site will not open in an IE or Firefox browser window. Instead they bring up a pop up window asking what to do with the files. I found that the reason for this was that Geocities was serving the files with incorrect MIME type headers.
A Geocities spokesman has stated that this is a deliberate policy because of pdf security vulnerabilities and that I could pay them lots more to have a web hosting plan that didn't cripple pdf files.
I think that this is something that Adobe should take up with Geocities.