Try doing something like this in the URL that serves the PDF.

http://www.myhost.com/cgi-bin/display_pdf.pl/something.pdf?key=value
[download]

The additional '/something.pdf' will show up as path_info in most cases, and all else will be as expected. The result is that the mac browser will likely see something.pdf as the filename and download it as such.

The problem is really that on on macs (pre-OSX), file extensions are for the most part ignored in favor of the resource-fork of the file (which surely your pdf will not have).