Hello Deborah,
All of the customers and vendors are held in the SY04905, with the EmailDocumentFormat being the type of document sent, with 1 being DOCX and 3 being PDF.
Here is a script that swaps everyone over to PDF:
UPDATE SY04905 SET EmailDocumentFormat = '3'
If you want to filter on Customers VS Vendors, then you can filter on EmailSeriesID. If you want to filter further there are other options:
EmailSeriesID – 3 (for Sales) & 4 (for Purchasing)
MODULE1 – 9 (for SOP), 11 (for Receivables), 12 (for POP) & 19 (for Payables)
Hopefully this gets you going in the right direction.
As always, have a backup before you do anything in SQL!