How PDF forms work

Interactive forms and pictures of forms

An interactive form carries field objects: text boxes, checkboxes, radio buttons, dropdowns, each with a name and a value. A reader draws them, lets you type, and stores what you typed separately from the page. The dominant flavour is AcroForm, defined in the PDF specification itself.

A flat document has the boxes drawn onto the page as lines and rectangles. There is no field, no name and no value — it is a picture of a form. Printing it and filling it in by hand is exactly what it was designed for.

The quick test: click where you would type. If a cursor appears, it is interactive. If nothing happens, it is not.

A scanned form is always the second kind, and worth its own note: scanning a filled-in form produces an image with no text in it at all, which is why sending one to a converter gives you a document full of pictures. OCR is what gives it a text layer.

Filling one in

For an interactive form, fill in a PDF form lists the fields it finds and lets you complete them. Because it works from the fields rather than from the appearance, what you type lands exactly where the form intends, at whatever size and alignment the field specifies.

For a flat document there is no field to fill, and the options are to print it, or to place text on the page with the PDF editor and position it yourself.

Two things that catch people out:

• Field values and the page are stored separately. Until a form is flattened, your answers live in the field objects. A tool that only reads the page content — including most text extraction — will not see them. This is why a filled form can look complete and come out empty when converted.
• Not every reader saves them. Some browser viewers let you type into a form and then save a copy without the values. Always reopen a saved form and check the answers are still there before sending it.

Flattening, and when to do it

Flatten PDF takes what the fields display and draws it permanently into the page, then removes the fields. The document then looks identical everywhere and cannot be edited back — the answers become ink.

Flatten before sending a completed form, for three reasons: it renders the same in every reader, the recipient cannot change the answers by accident, and the values are then real page content that extraction and search can see.

Do not flatten a blank form you are distributing, a form you may need to correct, or anything you want someone else to fill in — flattening removes the fields, so it turns a working form into a picture of one.

The order matters when signing is involved: fill first, then sign, then flatten. Flattening before signing makes the answers permanent but leaves the signature detachable; signing before filling means signing a document that was not finished. How to sign a PDF covers the rest.

One caution worth naming: flattening removes the interactive structure that assistive technology relies on to announce a field and its label. For a form someone may need to complete with a screen reader, that is a real cost — see what makes a PDF accessible.