Just off the top of my head...

COM Servers can't have any user interface / dialog prompt windows. If the call to the PDF writer is failing for some reason and trying to display a prompt window for user confimation, this would cause an error.

You may want to look into SYS(2335) -- server unattended mode control which will throw an error for any type of UI dialog prompts.

If your building with VFP8, you may want to look into error trapping with
TRY
CATCH
ENDTRY
which will supercede any ON ERROR trapping that may be defined.

Don