PDF not printing correctly

I am mail merging a letter with my CRM leads.

I have created a template, using arial font.

I merge and download a pdf of the merged letter - all is fine.

When I print, some letters are defaulting to a small box symbol.

Particularly bad is the D in the 'Dear $FIRSTNAME$

It don't default to the box everytime.

Also, occassionally some of the letters in the post print as the box.

Can anyone help with this?

David