winmail.dat and pdf

I have a user in Outlook 2000 on Windows NT. When he sends pdf docs to a client, she only gets the winmail.dat file. I had him resend the email in plain text format, and they still got the same thing. The strange thing is, his secretary (with the same Outlook and OS) *is* able to send the attachments to the same recipient and the attachments are received intact. I am out of ideas. Any help?