Flatten a PDF form

How to use it

1. 1

   Open the filled form

   Every field is listed with its type, its page, and the answer stored in the file. A summary counts how many are answered, empty or locked.

2. 2

   Read the warnings

   Required fields left empty, signature fields that will stop validating, tick boxes that may lose their mark, and comments sitting on top of the pages are each called out in words.

3. 3

   Correct any answer

   Type into the value column to fix a typo or fill a gap. Search and the Empty filter make a long form manageable.

4. 4

   Bake the answers in and download

   Each field's appearance is drawn into the page and the field itself is removed, so nothing in the download can be typed over.

Your PDF never leaves this page

The file is read into memory by your browser and rewritten there. There is no upload, no queue and no server copy, which you can confirm in the network tab: choose a file and watch nothing happen. That matters more for PDFs than for most things, because filled forms carry names, addresses, bank details and medical answers.

What flattening actually does to the file

An interactive form field is two things: a value stored in the field dictionary, and an appearance stream that says how that value should look. Flattening draws the appearance stream straight into the page's own content, then deletes the field and its widget annotation. The text you see stays exactly where it was, but it is now ordinary page content: no longer a field, no longer exportable as data, and no longer editable by whoever opens the file next.

The fields most likely to go wrong

Tick boxes and radio groups are the weak point. A checkbox stores its look as a small dictionary of states, one per option, and some producers never write the on state at all. Flatten a file like that and the tick simply disappears, which is worse than leaving the form alone. Any tick or radio in your file is counted and flagged for that reason, and the tool tells you afterwards which fields it could not bake in rather than pretending they worked.

Flatten first, sign afterwards

A certificate based signature covers an exact range of bytes in the file. Flattening rewrites those bytes, so a document signed before flattening reports as altered afterwards, even though nothing you can see has changed. The order that works is always the same: fill the form, flatten it, then apply the digital signature to the flattened copy. If the file already carries a signature, that is flagged the moment it opens.