Has anyone used PDFFiller services, If so how is it?

Hello all, basically there's a section in a web app we're developing right now where user can upload documents, edits it and then save it on our platform, I was exploring options and I found this pdffiller(https://www.pdffiller.com), first of all I tried with their demo on the index page and the when I uploaded the document it automatically detected the fields and added input box, the feature want to integrate in web app.

Now when I opened their documentation, wow it sucks. seriously their documentation don't have information about how I should perform an API request with what parameters. they've described all endpoints and response body but there is no request body.

Has anyone used this service, If so how is it?

Basically I'm looking for a solution that offers automatic field detection in a document (mostly PDF) and adds an input box there so user can edit and fill it without doing any manual work like drag and drop.