You attached a document in Purchase Order and cannot see the attachment in the Supplier Invoice.
Currently Purchase Order attachments are not copied into Supplier Invoice.
Supplier Invoice are receipts received from Supplier to be entered in the system.
Now the attachments in "Purchase Order" are generally information flowing from Purchaser to Supplier, hence this flow of attachments from Purchase Order to Supplier Invoice has not been considered.
This is the current system behavior.
KBA , SRD-CC , Cross Components , Product Enhancement