Does the PDF have a .pdf extension on it? If not, the email program could be seeing the document as a text file.
If you're doing Send Mail via SMTP, I don't think this is relevant, but I'll throw it out there anyway. Microsoft Outlook is infamous for sending attachments that, unless the recipient is using something like Outlook or Outlook Express, end up as winmail.DAT files and need to be converted before they can be viewed.
A fix is to disable RTF emailing in Outlook. But again, if you're not doing Send Mail via email client I wouldn't think this would enter in.
Ann Arbor, MI
It is Send Mail via SMTP so it is not using a mail client like outlook.