Hi there,
We have released an App on the HubSpot Marketplace that does something like this. It can make a copy of a document and add the document link to a custom property on the deal card. You can check it out here:
It also can take information from Deals and associated objects and place them into a Google Doc or Slides file (including loads of other HubSpot data) and has eSignatures built in.
Please let me know if you have any questions.
Cheers,
James