Optimize PDF · guide
What Flatten PDF does
Flatten PDF makes the interactive parts of a PDF permanent: filled form fields become part of the page with no box left to type in, comments, highlights and stamps are painted in, and hidden layers are cut from the pages and the layer switch removed, so they cannot be turned back on. If a file's layers cannot be cut out safely, they are left as they were and the result says so. Flattening does not scrub the file: the words of pop-up comments and parts of hidden layers can stay inside it. With Form values ticked, the saved file is reopened and checked, and if any form field is left you get an error, not a file. Links can stay clickable. Flattening happens in your browser; the file is never uploaded.
How to flatten PDF in your browser
- Choose one PDF Select the PDF with forms, comments or layers to flatten. It opens in this browser tab and is not uploaded.
- Choose what to make permanent Under Make permanent, tick Form values, Comments and annotations and Visible layers as needed. Keep working links, ticked by default, leaves links clickable.
- Flatten and save Select Flatten the PDF, read which parts were flattened, then save the new PDF. Keep the original in case you need to edit it again.
Worked example
Make completed form answers part of a reading copy.
- Starting point
- Choose a filled ordinary AcroForm and retain the editable original.
- What to do
- Under Make permanent, keep Form values ticked (the other boxes there are ticked by default too), select Flatten the PDF, then reopen the saved copy in a viewer.
- Result
- Expect the answers painted into the page and the fields gone. Read every answer, not just whether the fields still click.
If the result is not right
If an answer is absent, return to the original and check its appearance in a compatible form editor. XFA forms require a compatible application rather than this flattening workflow.
Limits worth knowing
- Flattening is not redaction or a security guarantee. Missing or stale field appearances can lose or misrepresent answers; links can remain clickable.
Questions and answers
Does flattening a PDF remove sensitive information?
No. Flattening makes what is visible permanent, but it is not redaction: form answers and comments become part of the page and can still be read and copied, and anything underneath a highlight or stamp is still underneath it. Hidden layers and the text inside pop-up comments stop showing in viewers, but some of that data can stay inside the saved file, where a PDF inspection tool can still read it. To remove sensitive text, use Redact PDF.
Why flatten a PDF before sending it?
So the recipient sees exactly what you saw and cannot change it: filled forms stop being editable, comments cannot be moved or deleted, and hidden layers cannot be switched back on. It also stops a form looking empty in viewers that draw form fields badly. A form field its designer set to hidden, such as an office-use box, is not dropped: it is painted onto the page with whatever it holds, so check the flattened copy for values you did not see before.
What happens if I flatten a digitally signed PDF?
The signature still shows on the page, but the digital signature stops working. A signature field is flattened like any other field and the file is rewritten, so a viewer can no longer confirm who signed it or that nothing has changed since. Flatten first and sign afterwards.
What happens to sticky-note comments?
The note's icon is painted onto the page where it was, but the words in its pop-up stop showing, because a pop-up is not part of the page. They are not wiped from the file, though: the words and the commenter's name can stay inside the saved copy. The result says how many comments had pop-up text. If you need what the comments say, copy or export it before flattening, and keep the original. If the recipient must not have them, delete those comments in your PDF reader before you flatten.
Why is part of my PDF still editable after flattening?
Two cases. A comment or markup saved without a drawn appearance of its own, which some apps leave for the viewer to draw, is left untouched rather than redrawn by guesswork, and the result lists it. XFA forms, an XML form format deprecated in PDF 2.0 (ISO 32000-2) but still used for some government and tax forms, are refused outright, because flattening their ordinary fields would leave the real form live in Acrobat; flatten those in Adobe Acrobat. Ordinary form fields are verified: if any survived, you would get an error, not a file. That check is for leftover fields, not for how an answer reads: a field that holds a value but has no saved appearance of its own is redrawn in Helvetica, so Cyrillic, Greek, Chinese and letters such as ł, ř and ş come out as question marks. Read each answer in the flattened copy before you send it.
Will links and bookmarks still work?
Yes, by default. Keep working links leaves link areas clickable, because a link draws nothing and flattening it would only break navigation; untick it to remove links too. Bookmarks are not touched, and nothing is turned into an image, so text stays selectable and searchable.