Thanks for your replies.

I am not using any anti-spam software. From the MIME headers it looks as
though Yahoo is using Cloudmark on their outgoing mail.

I haven't had a chance to do a test using a new identity in OE yet.

The message is structured as multipart/mixed and contains a
multipart/alternative section and the application/pdf section. The
multipart/alternative section consists of text/plain and text/html parts. I
have successfully received a similarly structured message from the same
sender containing a Word (.doc) file.

I noticed that in the message source display the MIME headers for the
application/pdf attachment are not highlighted in bold as all the rest are.

I also noticed that the pdf file had a long filename containing quite a few
spaces which I thought could have caused problems. However the sender tried
using a shorter filename but the result was the same.

Nick