I would suggest that a perfectly useful work flow would be .....

Raise purchase order, receive items...

Receive .....

• Scanned delivery note - Receive items make PO status "Received / Part Received" & attach scanned delivery note (If we convert to bill now we may not have had the bill yet & can't fill the relevant details forcing duplication of workflow ?)
• Scanned Bill - Select received/part received PO, convert PO to bill, fill with OCR details & attach bill.

And please allow quantity received to be different to PO quantities. Sometimes we receive more than the amount in the PO which is quite acceptable and other times we receive less, so an option should also be included to close the PO on bill receipt!