Extracting attachments via api on PO's and AP invoices associated with a project

Summary

How can I go about programtically extracting attachments via api on PO's and AP invoices associated with a project?

Content

After a project closes our project teams have to for a given PO:

1) Manually open each PO and download each attachment associated with the PO

2) Manually open each AP Invoice and down each attachment associated with it

3) After that they combine all of them into one pdf.

Looking for ways to accomplish 1) and 2) through external web service or API where after giving a PO number as input we can download all attachment associated with all Project PO's and all Project AP invoices.