Currently, when partially invoicing a Purchase Order, any attachments are ignored, and do not flow to the posted purchase invoice.
This means that if you want to attach a copy of the PDF of the invoice, you need to use the one incoming document available for the PO.
If you then get another partial invoice for that purchase order, there is no option to add an attachment or incoming document to the posted document, other than by opening the posted document and attaching it at that point.

It would also be really good to be able to attach a new incoming document to a PO every time an invoice needs to be posted.