How to flatten a PDF on a Mac
By Himanshu Ranjan · Updated 8 November 2026
To flatten a PDF on a Mac, select it and run Sign & Lock › Flatten forms to turn fields into plain page content, or Pages › Flatten annotations to bake comments and markup into the page. Each saves a new copy locally, so the original stays editable.
What flattening means
Flattening combines a PDF's layers into one, as Adobe describes it, so the content cannot be edited afterwards. In practice there are two common cases. A filled-in form has fields that remain editable until they are flattened. A PDF with comments, highlights and drawings keeps them as separate objects that can be moved or deleted.
FileDial offers a tool for each case, and it is worth choosing deliberately because they behave differently. Flattening is a way of fixing what you filled in; it is not a security measure. Anyone can still copy text from a flattened PDF if the text is still there.
Flatten a filled form
Use this when you have completed a form and want the answers to become part of the page, so the recipient cannot change them and every viewer sees the same thing.
- Select the filled PDF in Finder.
- Hold the right Option key to open the dial and choose the Sign & Lock category.
- Release on Sign & Lock › Flatten forms.
- Open the new copy beside the original and click a field to confirm it is no longer editable.
- Keep the original if you may need to correct the form later.
When to flatten, and when not to
Flatten when the document is finished and the recipient only needs to read it: an application form you completed, an invoice with markup, a drawing with review notes that must stay visible. Do not flatten while the document is still moving between people, because nobody will be able to fix a typo in a field.
Flatten a copy, not the only file. Keep the editable original in a folder named for the version, and send the flattened copy. If you later discover an error, you can correct the original and flatten again in seconds. A flattened file is a deliverable; the editable one is the source.
Flatten comments and markup
If a PDF has your notes, highlights or drawings, Pages › Flatten annotations makes them part of the page, as the tool describes. Form fields stay fillable with this one, which is the reverse of Flatten forms.
If you instead want to delete markup, use Pages › Remove annotations, which removes comments, highlights and drawings but keeps links and form fields. Know which you want: flatten to keep how it looks, remove to send a clean document.
- Keep the look, lock the form: Flatten forms.
- Keep the look, lock the markup: Flatten annotations.
- Remove the markup: Remove annotations.
Flattening is not redaction
A flattened PDF can still contain the text underneath whatever you covered with a drawing. Drawing a black box in markup and flattening it hides the text visually, but text beneath may still be selectable. For hiding sensitive text, use Sign & Lock › Redact text… or Sign & Lock › Burn in redactions, then verify with Sign & Lock › Check redactions.
Read our guide to redacting a PDF properly for why this keeps going wrong.
Several FileDial tools flatten links and form fields as part of writing the result: page numbering, headers and footers, watermarks, resizing and the stamps. If you need a fillable form, do that step last or on a separate copy. Convert › PDF to PDF/A also flattens links and form fields, because archive formats favor a fixed look.
Honest alternatives on a Mac
Preview does not have a flatten command that we can point to in Apple's documentation, so we will not give you steps for one. What Preview does offer is annotation, form filling and saving; the markup stays editable, which is often what you want while you are still working.
If you only need to stop casual edits, setting permissions is a different tool: Sign & Lock › Set permissions… opens normally while printing, copying and editing follow your choices. And if you need proof that nothing changed after you signed, a certificate signature is the answer: Sign & Lock › Sign with certificate… makes any later change show as broken. Flattening alone gives you neither.
Why do it offline
Forms usually hold personal details: names, addresses, ID numbers. Flattening locally keeps those off third-party servers. FileDial runs with Wi-Fi off and writes a copy beside the original.
After flattening, test the copy the way a recipient would: open it in Preview, click where a field was, and try selecting a line of text. If the result is not what you expected, delete it and run the other flatten tool on the original, since the two tools do opposite things with forms and markup.
Questions
What does flatten PDF mean?
It merges editable layers, such as form fields or comments, into the page itself so they can no longer be changed in a viewer. The page looks the same. Text may still be selectable, so flattening is not a way to hide information.
Should I flatten a form before emailing it?
Often yes, if you want the answers fixed. Run Sign & Lock › Flatten forms on a copy and keep the original fillable version. After flattening, the recipient cannot edit fields, though they can still read and copy the filled text.
Does flattening make the PDF smaller?
Not reliably. Flattening may change the file size slightly in either direction, because a new file is written. If size is the real goal, compress as a separate step with Compress › Compress PDF, which keeps text selectable while images go to 144 dpi, and compare the results.