Is this web form to pdf or pre existing fillable pdf to json?
If you upload a scanned PDF (e.g. plain images), then you can add form fields in the template editor. If you upload a pdf with a fillable form, then we will automatically import all of the existing fields (and the imported fields can then be modified or removed.)
If I understand it, create an HTML form, submit to server, spit back PDF. This has been built into ColdFusion for about a decade, and it is trivial. [and I assume other server side software can do the same thing].
Aside from that, the bulk of the PDF Forms I download these days can already be filled out with Acrobat. I'm not sure what benefit you're offering me in time savings.
Or if I built a service for end-users to fill out and sign PDFs, I would create a separate website and use FormAPI to generate the PDFs. (I don't think I will do this, though.)
You're right that if you just need to fill in a single form, then there are easier ways to do that. (Even just using the Preview app on Mac).
I guess I'm not understanding the niche you fill.
Good luck with it, though.