What flattening a PDF does, and what it never protected

Flatten a PDF

Drop the file above and choose Flatten. It runs on your device, so nothing is uploaded, and you get a copy with “-flat” in its name; your original stays as it was. In the editor, the save menu offers the same as Save flattened PDF, next to Save PDF, which keeps fields editable.

What flattening does to the file

A filled PDF form is 2 things on top of each other: the printed page, and a layer of interactive objects (fields, and annotations such as highlights and notes) that a viewer draws on top and lets you change. Flattening asks the engine to draw that layer into the page the way it looks right now, then removes the interactive objects and the form itself.

We flattened an invented application form: a name field filled with “Jane Cooper”, a ticked checkbox, a country dropdown set to “Austria”, a highlight over the heading, a sticky note saying “Check clause 4 before sending”, and a black rectangle drawn over the word “Country”. Flatten reported “Flattened 5 fields and annotations on 1 page into page content” in under 0.1 seconds. What changed:

• Fields and annotations: 5 before, 0 after. Inspect now says “Form: no form”.
• The filled values became page text. Before, “Jane Cooper” and “Austria” were field values: select-all on the page did not include them. After, select-all copies “Jane Cooper Austria” with the rest of the page.
• The highlight and the note icon are drawn on the page; the note’s words are gone. Flatten removes the note itself along with anything nothing points at any more, so “Check clause 4 before sending” was nowhere in the flattened file’s bytes (we decompressed and searched all of them). Only the icon is left, so delete the note in the editor first if even that must not show.
• The word under the black box was still there. The box was a drawing on the page, not a field, so flattening did nothing to it, and “Country” copied out from under it.
• In getPDF’s editor, the flattened values are 1 grouped object. You can move or delete it as a block, but not retype the name inside it. Make a PDF editable covers what to do with a flattened form you need to change.

What it protects against, and what it does not

8 things people hope flattening prevents, tested against what it does:

And 1 more that is not on the list because flattening never touched it: who can open the file. A flattened PDF is as readable as before. To keep strangers out, set a password to open with Protect PDF, which encrypts the file with AES-256; the privacy and protection guide explains the 2 kinds of PDF password.

Flattening is not redaction

This is the expensive confusion. People draw a black box over an account number, flatten the file “to make it permanent”, and send it. The box is now part of the page, and so is the account number underneath, which anyone can select, copy or search for.

Redact PDF does the opposite: it takes out the characters, picture pixels, drawings, links and form fields under each box you draw, and then draws the box. After a redaction there is nothing under the box to flatten; getPDF’s redaction already writes a new file without the removed content. White-out versus a real edit shows the difference, measured on our invoice test file.

The signature case

People often flatten “to protect the signature”. What that actually changes depends on how the signature got there:

• A signature placed with getPDF’s editor (drawn, typed or a picture) is written as ordinary page content, lines or a picture, not as an annotation. It is already as flat as it gets; flattening changes nothing about it.
• A signature placed as a stamp annotation by another app becomes page content when flattened, so it can no longer be dragged off as 1 object. A screenshot still copies it.
• A certificate-based digital signature is a different thing: a cryptographic seal over the file’s bytes that shows whether anything changed after signing. Flattening rewrites the file, so it breaks that seal, and getPDF warns you whenever a tool writes a new copy of a signed file.

What makes a signature hold up, and when a drawn one is enough, is covered in the signing and filling guide.

When to flatten

• Before sending a completed form to a person or a portal, so your answers display the same for them and nobody changes them by accident.
• When a recipient sees empty fields that are filled for you: some viewers do not draw fields well; flattened values are page content, which every viewer draws.
• Before printing from an app that prints fields badly.
• Before archiving a filled form, so it reads the same in 10 years without relying on form support.

And when not to: when the recipient must fill in more fields, when the file carries a digital signature that must stay valid, and when you have not kept an unflattened copy. Flattening cannot be undone.

What flattening does not do in getPDF

Flatten PDF handles AcroForm fields, the common kind, and all the usual annotations. It does not flatten XFA forms, the older Adobe-only format some government forms use: it warns when it meets one and suggests filling it and printing it to PDF in Adobe Reader instead, and Inspect shows those as “Form: XFA (not editable here)”. It does not remove attachments, hidden layers or document properties; hidden pages and attachments covers those. And it does not make content unreadable, which is the job of a password for the whole file and of redaction for parts of it.