It sounds like you want to fill out a pre-generated PDF? It's a lot easier to do that server side.
You can use pyODK to pull the submissions from Central and PyPDF2 to fill out a PDF template.
You could also try using the ODK Zapier Integration to connect ODK with PDF.co and build a flow where all approved submissions generate a PDF.
Is there a reason why this needs to happen on the mobile device and not the server?