I am able to successfully attach my PDFs to my outgoing mails and receive them as well. No issues at my end..
Maybe the issue is with the PDF you are trying to attach. Can you please try attaching another file and see if that works... My apologies if this suggestion is too vague..
Tried multiple PDF's and they all lockup the program. I even tried PDF's that I did not create and still no go.
It must have been something in the installation. I removed and then re-installed the Reader and everything is now fine.