According to the SAP forums.
No. This is not possible.
“It is currently not possible to send attached documents in a purchase
order for email or SAPmail. The technical reason is: During the output,
the form (e.g. MEDRUCK) is already handelled as an attachment.Therefore,
it is not possible to attach documents together with the form.
You could send a purchase order (PO) document via email “external send”
as body text or to convert this PO as PDF attachement. To add aditionnal
attachments in the same mail is not supported from the application side.
Therefore, for manual use, you could send the external mail to your
mailbox and add any attachements before sending the email.