This would be for documents NOT created/generated within GHL. Popular form software (such as GravityForms) can generate full PDFs of the form submissions and links to download those PDFs can be passed to GHL via webhook. I admittedly don't know if there are limitations that would prevent it, but if possible being able to upload a document to a contact card via a URL would be exceptionally helpful.
Another use case I can think of would be if you have a form that asks the end-user to upload a document, if that could be added directly to the contact card via automation, which would again be passed into GHL via a URL.
We are currently adding these URLs to Notes, and then manually downloading them/adding them to the 'Documents' section on the contact card.