Prevent copying of e-mailed PDFs

We create mail merged e-mails with letter heads and signatures.
The automatic e-mail merging and sending works fine.
However, we need to prevent copying of the e-mailed attached PDF files. Printing is permitted!
We set the Preferences / Security to restrict editing and prevent copying of text and images (with password set). The are security features are set when creating the PDF e-mail merges from Word. The e-mailed documents DO NOT have the editing security features enabled. Any part of the e-mailed files can top selected and copied. How can we lock the edit security in the e-mailed documents? Thanks.