Are you certain that it's able to be saved. I've had a number of PDF files that allowed me to fill out form fields in them, but only for the purpose of printing them when completed. You could not save the file with the form fields filled in.
There are others, of course, that do allow you to save them with the data filled in. My Federal and state Tax forms are like this.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore