The only way to “attach” a PDF from a different form is to add hidden fields to FORM B that are populated by using Formidable’s “Get Value from Entries” shortcode. Then create a field map layout from FORM B to merge to the FORM A PDF…
Alternatively, you can place a link in FORM B’s email message to FORM A’s PDF, which is much easier.
Let us know if you need more assistance with these options.
We remain at your service.