Question marks instead of a

We use a network printer and now have the problem that a user print some PDFs (were all created in the same way) have missing letters which are filled with question marks (misses some a (in another document the o)). This happened even after re-create the PDFs, the original Word file is printed correctly so on other Computers, other PDFs go without problems. Printer drivers are all new. hope you can help.

used: Adobe Acrobat 8 Pro