Outlook Express now comes with an anti-viral feature which blocks access to executables – quite a nifty feature if you"re bombarded by viral attack.
But a side effect of the "feature" is that Outlook Express defaults to deny access to PDF files, prompting some to suspect there"s an ulterior motive along the line somewhere.
Microsoft has had its beady eye on Adobe for quite a while, and is understandably miffed that Adobe and the PDF format is so widely accepted.
It has its own XDOCS specification but it will take a while for that to be successful, if it ever is.
The anti-viral feature can be turned off by going to Tools > Options and (under the Security tab) unchecking "Do not allow attachments to be saved or opened that could potentially be a virus".