I think Doc Attach has the potential to be a killer feature both on its own and as a gateway to more sophisticated document management systems. Checkout Microsoft Dynamics GP 2013 R2 Feature of the Day: Document Attach – Payables
Microsoft Dynamics GP 2013 R2 Feature of the Day: Document Attach – Payables – Inside Microsoft Dynamics GP
Tips & Hidden FeaturesPayables & ReceivablesGP 2013
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.