# How to fill in a PDF form (and when you cannot)

> Which field types a PDF can hold, what flattening really does, and why a scanned PDF cannot be filled in. Tested with a real form.

Source: https://toolpantry.app/blog/how-to-fill-in-a-pdf-form

Filling in a PDF form is typing into the boxes the document already has. Knowing which boxes those are, what flattening does to them, and why a scanned PDF has none at all saves most of the frustration.

## Quick answer

**Short answer:** open the [PDF form filler](https://toolpantry.app/fill-pdf-forms) and every field in the file is listed for you. We built a test form with five fields of four different types, filled it in, and read the file back: the text field held **Ana Ruiz**, the checkbox **/Yes**, the radio group **/basico** and the drop-down **MX**. What you **cannot** do is fill a PDF that has no fields — a scan or a flat document — and the tool says so plainly instead of producing a broken file.

## What a fillable PDF actually is

A fillable PDF is not a picture of a form. It is a PDF with **real form fields** inside it, each with a name and a type. When you open it in a reader that understands forms you get a box you can type into; when you open it in a program that does not, you only see the text that happens to be inside the boxes.

That difference explains the two most common frustrations: a form that will not accept typing (it has no fields, it is a scan), and a form where you can type but the text does not move (the program you used only reads it).

## The field types, and how each one is stored

| Type | How it looks | What lands in the file |
|---|---|---|
| Text | A one-line box | Free text |
| Checkbox | A small square you tick | Checked or not |
| Radio group | A drop-down with the choices | One of the options |
| Drop-down | A menu with the PDF’s own options | One of the options |

We built a form for the test with reportlab — two text fields, a checkbox, a radio group and a drop-down — ran it through the [PDF form filler](https://toolpantry.app/fill-pdf-forms) in the browser, and then read the resulting file back:

| Field | Typed | Read back from the PDF |
|---|---|---|
| nombre | Ana Ruiz | Ana Ruiz |
| email | ana@example.com | ana@example.com |
| acepto | ticked | /Yes |
| plan | basico | /basico |
| pais | MX | MX |

All five values are written into the PDF itself. Nothing leaves your browser.

## What the flatten checkbox does

This is the difference between a live form and a finished document:

- **Without flattening**, the fields stay alive. You can reopen the PDF and change anything. That is the right choice while someone still has to review or sign it.
- **With flattening**, the fields **disappear** and the values are drawn onto the page as content. We checked: after flattening, the PDF has zero form fields left, and the page text reads “Formulario de prueba Ana Ruiz ana@example.com MX”.

A flattened PDF can no longer be edited field by field — it is a picture with the answers painted on. That is exactly what you want before sending it, and exactly what you do not want if the document is going to come back to you.

## When you cannot fill it

The common case: a PDF with **no fields at all**. It is either a flat document (text drawn on the page) or a scan (the page is an image). The tool says so rather than handing you a broken file:

*“No fillable form fields found — this PDF may be a scan or a flat document. To sign a flat PDF, use the sign tool.”*

If it is a **scan**, the text is an image, so there is nothing to fill: it has to go through OCR first. That is what [the OCR tool](https://toolpantry.app/ocr) is for, and [PDF to PNG](https://toolpantry.app/pdf-to-png) gets you the pages as pictures if you need to read them by eye.

## Filling is not signing

Different things, different consequences. **Filling a form** means typing into the boxes the document already has. **Signing** means adding a signature: the drawn one you make with your finger, or an electronic signature backed by a certificate. If what you need is a signature, use [the PDF signer](https://toolpantry.app/sign-pdf) — and our guide to [signing a PDF](https://toolpantry.app/blog/how-to-sign-a-pdf-on-iphone) explains what the three kinds of signature actually mean.

## The limits, plainly

- **Only PDFs that already have fields.** The tool does not invent a form where there is none.
- **Unusual field types are left alone** and shown as not editable, instead of breaking the file.
- **Filling validates nothing.** It does not check that the email address exists or the ID is real; it writes what you type.
- **Nothing is encrypted.** If the original was protected, the values still get written, but the file is not newly protected.

## Notes

Written by the Tool Pantry team on 22 October 2026. Everything here was measured on a purpose-built form (two text fields, a checkbox, a radio group and a drop-down) run through the tool in a real browser, with the output PDF read back afterwards to confirm what actually landed in the file. For the signature side of this, see our [guide to signing a PDF](https://toolpantry.app/blog/how-to-sign-a-pdf-on-iphone).

## FAQ

**How do I fill in a PDF form?**

Open the PDF form filler , choose your file, and the page lists every field with its type. Type into each box, tick what applies, then press Save. The values are written into the PDF inside your browser — the file is never uploaded.

**Why does my PDF have no fields?**

Because it is not a form: it is either a flat document (just text) or a scan (the pages are images). The tool tells you when it finds no fillable fields. A scan needs OCR first, and PDF to PNG exports the pages as pictures if you would rather read them by eye.

**What is the difference between flattening and not?**

Without flattening the fields stay in the file, so you can reopen the PDF and change them. Flattening removes the fields and draws the values onto the page: the document can no longer be edited field by field, but nothing can be changed by accident either.

**Are the values really saved inside the PDF?**

Yes. We checked with a purpose-built PDF: after filling it in and reading the file back, the text field returned “Ana Ruiz”, the checkbox “/Yes”, the radio group “/basico” and the drop-down “MX”. The filled file was 9,451 bytes against 11,650 for the original.

**Is filling in a form the same as signing it?**

No. Filling means typing into existing fields; signing means adding a signature. If you need a signature, use the PDF signer .

**Is my PDF uploaded anywhere?**

No. The fields are filled and saved inside your browser, and the file is downloaded from the same tab.