Adobe Paperless issues

I work for a transit authority and we have to do things by the book to receive federal funding.

We are trying to go paperless..  A po is created, sometimes a packingslip or Invocie is present when it goes for signature, sometime not. When we send a PO, and a 3 price bid to the proper authority, in Adobe sign it does not allow an attachment of a PO, packingslip, invoice or check.  Is there a way to take a signed document (PO) and attach an invoice etc? Even if it becomes a new document? Or link it to an invocie for example?