Email send with .pdf is converted to .dat

Clarifications

adobe reader to be able to read the .pdf attachment?

http://support.microsoft.com/kb/278061

What might be happening is really a combination of an email security app stripping the attachment and the link I posted above.

How long is the attachments file name?
There is a max, exceed it and you'll get ATTxxxxxx.dat at the other end.
Force the email to go in plain text, does that work?