Answer
There's no direct configuration for attaching documents to the Print-Dialog-generated email specifically, but a workable combination of existing features gets there: send the attachments through a PDF_REPORT_CREATED custom event instead (pulling the recipient's email address from the Purchase Order's supplier contact), then suppress the redundant Print-Dialog email using a Report Rule with the SkipJobEmail action property so it doesn't also go out separately. Since the suppressed Application Message still lands in the OUT1 queue rather than actually being blocked from creation, add another custom event to move it from OUT1 into the TRASHCAN queue, and set up a Scheduled Database Task to periodically clean out TRASHCAN messages (e.g. hourly) so they don't accumulate.