How to fill in a PDF form, even one that isn't fillable

There are two kinds of PDF forms, and they look identical until you try to click in a box.

A fillable form has real fields built into the file: text boxes, tick boxes, drop-down lists. A flat form is a document that was designed to be printed; the boxes are just lines drawn on the page. You can't type into a line, so flat forms need a different approach.

Fillable forms

1. Open Fill form and add the PDF.
2. The tool reads every field and shows them as a list: text boxes, tick boxes and choice lists, with their names.
3. Fill them in. Field names are sometimes cryptic (txt_12b is a classic); compare with the page if you're unsure which is which.
4. Tick Lock the answers after filling (flatten) if you're sending the form to someone who shouldn't change it.
5. Press Save form.

If the tool says the PDF has no fillable fields, you have a flat form.

Flat forms

For a quick answer or two, Add text places a line of text at one of nine positions. For a real form with many boxes, use PDF Studio, which lets you click anywhere:

1. Open the PDF in the Studio.
2. Add a text box, type your answer, and drag it onto the line. Use the Small print style or set the size to around 10 pt so it matches typical form text.
3. Duplicate the box with Ctrl+D for the next answer; it keeps the same style.
4. Use the Tick element from Elements for check boxes, and Draw to sign.
5. Download the PDF.

Why flatten?

A filled-in fillable form still contains live fields, so anyone receiving it can change your answers, sometimes by accident when they open it. Flattening turns the answers into ordinary page content. Do it when you send a final version, and keep an unflattened copy for yourself in case you need to correct something later. You can also flatten separately with Flatten PDF.

A small courtesy

Name the file something useful before you send it back: lastname-application-2026.pdf beats form(3)-final-FINAL.pdf for whoever has to file it.