Disappearing attachments

When I reply to an email with an attachment I find that the attachment disappears when I next read the email. What am I doing wrong please?

Hello Gordon,

Can you tell us what type of attachment does this issue happen with? Please contact me at hester@emclient.com with a link to this thread for a fix.

Regards,
Maurice

Hello Maurice,
The type of attachment I have specifically noticed disappearing is PDF.  There may be others that were of no particular importance so I failed to notice.
Kind regards,
Gordon