How do you flatten a PDF?

What flattening does

A filled PDF form usually stores your answers twice over. There is the field value — the string AB12 3CD attached to a field named postcode — and there is the appearance stream, a small piece of drawing instruction that renders that string inside the box. The page content itself, the part that was there before you typed, does not know about either.

Flattening collapses that. The appearance is drawn into the page content, the field is removed, and what is left is a page that says AB12 3CD in the same way it says everything else printed on it.

	Unflattened form	Flattened PDF
Answers can be edited	Yes	No
Renders the same in every reader	Not reliably	Yes
Fields can be re-read by software	Yes	No
Signature can be dragged off	Yes, if it is an annotation	No
Text is selectable	Yes	Yes

When you actually need it

When you are sending a form back. The recipient should receive an answer, not an editable draft with your answer in it. This is the ordinary case, and it is why the export here flattens by default.

When the other side says it looks blank. The most common support complaint about filled PDFs. Some readers — phone viewers especially, and email preview panes — do not regenerate appearance streams, so a form full of answers arrives looking pristine. Flattening ends that class of bug for good.

When a portal rejects the file. Plenty of upload systems refuse PDFs containing form fields or JavaScript outright, or strip them and lose your answers in the process.

When you have signed it. A signature left as an annotation can be moved or deleted by whoever opens the file next. Drawn into the page, it cannot.

When you should not

Flattening is one-way. Keep the unflattened copy if the form will come back for a correction, if it is a template you reuse, or if anything downstream reads the field values as data. And if the document needs a certificate signature — a cryptographic seal rather than a picture of your name — flatten first and sign second, because flattening rewrites the bytes that a certificate signature covers.

Flattening it in the browser

FormworkPDF flattens on export. The mechanics are worth stating because they determine whether your answers survive:

1. The original bytes are kept untouched in memory from the moment you open the file, and every export starts from them again. Filling is never destructive.
2. Values are written into the real fields, and anything you placed by hand — text, a signature, initials, a stamp — is drawn at its stored position in PDF points.
3. Answers on blanks that were detected rather than declared have no field to write into, so they are drawn straight onto the page as text. They read back out of the finished PDF as text, not as a picture of text.
4. The form structure is removed and the file is written out.

All of it happens inside the tab. pdf-lib does the reading and writing in a Web Worker, so a 200-page document does not stall typing, and nothing is uploaded at any point.

The one that bites people

Rasterising a PDF — which is what every compressor does to hit a size target — renders each page with annotations disabled. An unflattened form keeps its values in annotation appearance streams. Compress an unflattened filled form and you get a file of exactly the right size with every answer missing, and no error anywhere to tell you.

That is why shrinking for an upload limit forces a flatten first. It is a silent failure otherwise, and a silent failure on the form you are about to send to a tax office is the worst kind.