Inserting PDFs into Word documents, as a mailmerge would imply, results in low-resolution images of the PDFs being inserted. They may print out a high-res on a postscript printer (not sure), but they don't do that even if 'printing' the Word document to PDF via a PDF driver. You would do better to create high-res TIFF or EMF/WMF images of the drawings and let the mailmerge use those.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]