Compress it and add the compressed file as an enclosure.
For example, control (right) click on the desired pdf in the Finder to bring up the Finder's contextual menu and then select Compress. It will make a .zip file of the pdf (it doesn't change the pdf). Then drag the .zip to the mail message. The receiver will get the .zip and they can expand it back to the .pdf on their end (if it's a mac then they can just double click the .zip to expand it).