Flatten PDF

A PDF can contain interactive layers: form fields you can type into, sticky-note annotations, freehand markup, and digital signature widgets. Flattening burns those layers into the page itself, turning them from editable objects into static drawn content. The text in a form field becomes ordinary text on the page; the tick in a checkbox becomes a tick-shaped vector path. Once flattened, no one can clear the values, edit the comments, or reuse the form.

This matters when you are submitting a final version. A solicitor sending a signed contract back to the other side does not want the recipient able to retype the witness's name in the form field. A teacher sharing marked-up coursework feedback wants their comments locked in place. Flattening is the safe, irreversible way to publish a finished document.

These three operations sound similar but solve different problems. Flattening removes the editability of fields and annotations. Locking with a password (via [Protect PDF](/protect-pdf)) controls who can open or modify the file. A digital signature cryptographically proves who issued the file and detects later tampering. For a sensitive handover, you often want all three: flatten the form, sign it, then password-protect the result.

Worth knowing: a flattened PDF is much harder to reverse-engineer. Some PDF editors will let a user OCR a flattened page and rebuild rough form fields, but the original field metadata, validation rules, and tab order are gone. For most practical purposes, flattening is treated as final.

The flatten step runs in your browser using pdf-lib. The PDF never leaves your device, which is essential when the form contains personal data: National Insurance numbers, dates of birth, bank details on a direct debit mandate. Free flattening services that upload to a server are a real privacy risk for these documents. The trade-off is that very large interactive forms (200+ fields) can be slow to flatten on lower-end devices.