How to make a PDF fillable, and why most fillable PDFs are not
A box drawn on a page is not a field. What a real form field is, how readers find it, and how to add them in one request.
Half the fillable PDFs in circulation are not fillable. They are a scan, or an export from a word processor, with rectangles where the answers go. Open one in Preview and you can type over it with the annotation tool. Open the same file in Chrome and you cannot. Email it to someone on a phone and they print it, write on it and photograph it back.
What a real field is
A real form field is an object in the PDF with a name, a type and a position. Text, checkbox, radio, dropdown, signature. The standard is called AcroForm and every major reader supports it. A field is why Acrobat, Preview, Chrome, Edge, Foxit and the phone readers all show the same blue box in the same place and all save what you type into it.
Because a field has a name, software can find it. That is what lets a form filler put the right value in the right place without a human clicking each box, and what lets a document check report which required fields are still empty.
Three ways to add fields
In Acrobat, use Prepare Form. It auto-detects likely fields from lines and boxes and lets you fix the ones it gets wrong. It is the right tool if you do this all day.
In BettrPDF, Fill & Sign types into fields that already exist and lets you click anywhere on a flat PDF to type, so most people never need to make the form fillable at all. Automatic field creation for senders who want a reusable form is on the roadmap.
For a scan, field detection needs real text to read the labels, which means OCR first. That is on the roadmap too.
What to check before you send it
Open the finished file in a second reader. If a field shows in Acrobat but not in Chrome, it is probably an annotation, not a field. Tab through the fields: the order should follow the page. And give every field a name a human can read, because that name is what the filler and the checker will use.