adding data from JSON dataset to an existing PDF document automatically saving a copy for sharing

Hi, 

I'm new to this community so forgive me if I'm asking to wrong forum.
We have a bunch of PDF documents that people are filling in manually - what we've been asked is can this be automated. I've already been down the web form route, and although I like the result, I have to pass all the data in via the URL, and this only works for a 1-1 , not a 1-n scenario.
I've done some research into Power Pages with Power Automate too , but this seems to only allow merging of data into a Word Template.

Ideally, what we would like to do is: From with our in-house web application, click a button/link and a request would be sent with all the necessary data in a JSON payload - this would be merged into a pre-existing PDF, and saved into a cloud repository , like sharepoint, and a URL returned. 
Hopefully, we can then allow add e-sign onto this, so the document can be shared with the customer and they can e-sign it.
What I'm struggling to grasp is the high level parts required to achieve this. Regarding the repeating 1-n data, I have considered whether this could be a separate document that is then merged with the base document. I would prefer to avoid recreating the PDFs in Word, if at all possible.

any pointers, gratefully received.