Users of Apple Mail may experience from time to time email attachments
"disappearing" when sending to non-Apple Mail using recipients.  After
running into this problem myself numerous times, and getting reports
from clients with the same problem, I decided to do a little research.
I won't go into technical whys and hows, but here's the solution.
When
composing a new email that you intend to attach a file to, always type
the body of the message, or at least some of it, BEFORE attaching the
file.  I have a habit of making a new message, and immediately
attaching an Invoice, then typing the message, so it's taken some
getting used to.  So far so good! 
Remember:  TYPE then ATTACH

Here's the source of the information if you want the details.