You will need to do the merge before saving the certificate as a .pdf.  Use
a formletter type mail merge.  If you want to create individual .pdf files,
see the "Individual Merge Letters" item on fellow MVP Graham Mayor's website
at:

http://www.gmayor.com/individual_merge_letters.htm

If you are using Word XP or later, the "Add-in to Merge Letters to Separate
Files" that I have written and that can be downloaded from that site will
allow you to create each letter as a separate file with a filename taken
from a field in the data source with a minimum of fuss and it can also
create a .pdf file of each.