Went to look up what XFA forms were (…adobe.com/…/pdf-forms-and-documents).

Most of the non-fillable forms I encounter are what that document lists as “Traditional” PDF forms, likely generated using older tools from print streams. For example, a school athletics release form, or a membership application for a small organization. None of them have any fillable PDF fields. The original expectation might have been to download and print out the PDF, hand-fill it, then fax the result back.

I’ll dig up a form like that I had to fill a few weeks ago and give it a try.