Fill in a PDF form in your browser and save it — optional flattening

How to use the PDF form filler

1. Drop a fillable PDF onto the box or press Choose a file. The tool lists every field it finds, for example 3 fillable fields found.
2. Type into text boxes, tick checkboxes and pick options from the lists. Each field is labelled with the name the form’s author gave it. Read-only fields are shown but cannot be changed.
3. Tick Flatten — make the answers part of the page (no longer editable) if you are sending the final version.
4. Press Fill form and download the result, named with -filled added.

What it does and when to use it

Many government, school, bank and HR forms come as fillable PDFs. Some phone and browser viewers show the fields but cannot save what you type, or save it in a way that disappears in another app. This tool fills the real form fields and writes their appearance into the file, so the answers show up in any reader.

Use flattening when the recipient should not be able to edit the answers, or when a portal rejects forms with active fields. Leave it off when someone else still has to complete part of the form.

How it works

When you drop the file, pdf-lib (@cantoo/pdf-lib) opens it in your browser and reads the form: each field’s name, type (text, checkbox, dropdown, radio group or list), current value and choices. These become the inputs you see. Buttons and signature fields are named under Not fillable here.

When you press Fill form, only the fields you changed are sent to a background worker. Text is checked against the Helvetica font’s character set and cut to the field’s maximum length if it has one. Checkboxes are ticked or unticked, and options are selected. The field appearances are then redrawn so every reader shows the same thing. With Flatten ticked, the fields are merged into the page.

Worked examples

A three-field application. A form with Full name (text), Agree (checkbox) and Country (dropdown with India, Kenya and Spain). You type Asha Rao, tick Agree, choose Kenya and tick Flatten. The summary reads 3 fields filled, form flattened.

Characters the font cannot show. Typing Ä Ł 你 in Full name gives 1 field filled and the warning 2 character(s) are outside the standard PDF font and were replaced with “?”. The Ä is kept; Ł and 你 become question marks.

Limits and tips

• Line breaks typed into a multi-line box are kept. A single-line field cannot show them, so there they become spaces.
• Calculated fields and form scripts (for example totals that add up automatically) are not run.
• Password-protected forms must be unlocked first with Unlock PDF.
• Check which fields a form has without filling it by opening it in the PDF metadata viewer, which shows the form-field count.