PDFWren

Flatten a PDF

Merge form fields into the page, or rasterise everything so annotations and layers cannot survive. Runs in your browser with nothing uploaded.

🔒 Runs in your browser

Two tools wearing one name

"Flatten this PDF" is one of those requests where the person asking and the person doing usually mean different things, and nobody finds out until afterwards.

Meaning one: lock the form. You filled in a form, and you want to send it back without the recipient being able to change your answers or accidentally clear a field. The values need to become part of the page.

Meaning two: strip everything. You want a file with no annotations, no comment threads, no layers, no hidden text, nothing but what is visible. This is what people mean when they say "make it like a scan".

These need completely different operations, and the second one is destructive. So both are offered here, labelled, with the cost of each stated before you commit.

Form fields only — the one you probably want

Form fields, checkboxes, radio buttons and signature fields are merged into the page content. After this, the values are drawn onto the page rather than stored in an interactive layer.

What you keep:

  • The text is still text. Selectable, searchable, copyable, and perfectly sharp at any zoom or on any printer.
  • The file usually gets slightly smaller, because the form structure goes away.
  • Everything else is untouched.

What you should not assume: this does not remove annotations, comments, metadata or anything hidden elsewhere in the document. It flattens the form. That is all it claims to do.

We verified it does what it says rather than trusting the library: after flattening a document with a filled text field, the output reports zero remaining form fields and the same page count.

Flatten everything — real, and genuinely destructive

Every page is redrawn as an image at 150 DPI and rebuilt into a new PDF. Nothing from the original structure survives, because there is no structure left — only pixels.

This is the only version that truthfully removes annotations, comment threads, optional-content layers, embedded scripts and text hidden under an image. It is also the version that costs you the most:

  • Text stops being text. No selecting, no copying, no searching, no screen reader.
  • The file usually gets bigger, often several times bigger. A page of vector text is cheap; a photograph of that page is not.
  • It cannot be undone. Keep your original.

150 DPI is the deliberate choice. It stays readable in print, where 72 DPI does not, and it avoids the four-fold size increase that 300 DPI would cost for a difference almost nobody can see on a text page.

The page-size trap, and why it matters

There is a specific mistake that browser-based flatteners make often enough to be worth naming.

Rendering an A4 page at 150 DPI produces an image 1240 pixels wide. If the tool then builds a new PDF page 1240 points wide — treating pixels as points — the result looks perfect on screen and prints at roughly four times the intended size, spread across several sheets.

This tool rebuilds each page at its original dimensions in points, and the check that it does so is part of the test suite rather than something anyone has to remember.

If your goal is redaction, stop here

Rasterising a page removes text hidden behind an image. It does not remove text you have covered with a black rectangle in a previous step, because a black rectangle plus text underneath renders as... a black rectangle. The pixels are safe, but only because you happened to cover it before flattening.

That is a fragile way to protect anything. If you are removing sensitive information, use redaction that deletes the content, not a sequence of steps that happens to hide it.

This tool runs entirely inside your browser. Your file is never uploaded — it never leaves your device, so there is nothing for us or anyone else to read, store or leak.

Common questions

"Flatten" seems to mean two different things. Which one is this?

Both, because it genuinely does mean two things and most tools pick one silently. "Form fields only" merges filled-in answers into the page so they cannot be edited, and the text stays selectable. "Flatten everything" redraws each page as an image, which is the only version that truly removes annotations, comments and layers — at the cost of the text no longer being text.

Which one do I need to email a filled-in form back?

Form fields only. It stops the recipient from accidentally — or deliberately — changing your answers, keeps the file small, and keeps the text searchable. Rasterising is overkill and makes the file bigger.

Does flattening remove hidden or deleted content?

Only the "flatten everything" mode does, because it discards the entire original structure and keeps pixels. Flattening form fields leaves the rest of the document exactly as it was, including any annotations, metadata or content you cannot see.

My PDF has no form fields and the tool refused to run. Why?

Because handing you back an identical file and calling it a success would be dishonest. If there are no fields, field-flattening has nothing to do. If you are trying to lock down annotations or layers, use "flatten everything" instead.

Will flattening make the file smaller?

Flattening form fields usually makes it slightly smaller. Rasterising almost always makes it larger, sometimes several times larger, because a page of crisp vector text is far cheaper to store than a picture of that text.

Last reviewed