What does it mean to flatten a PDF, and when should you do it?

Flattening a PDF means turning the things that sit on top of the page, such as form fields, comments, highlights and stamps, into ordinary page content. The file looks exactly the same afterwards, but those parts can no longer be clicked, edited, moved or deleted as fields or comments. You can do it with our Flatten PDF tool in your browser.

People usually flatten a filled form before sending it, or a marked-up document whose comments look different from one app to the next. It's useful, but it's often mistaken for security, which it isn't. More on that below.

A PDF page has layers

Think of a PDF page as a printed sheet with transparent stickers on top.

• The page itself holds the text, lines and images that were there when the document was made.
• Form fields are the stickers you type into: text boxes, checkboxes, radio buttons, dropdowns.
• Annotations are the stickers added later: comments, sticky notes, highlights, underlines, drawings, stamps.

Each sticker carries its own information and often its own description of how to draw itself. A PDF reader draws the page first, then the stickers. Different readers don't always draw them the same way, which is why a filled form can look perfect in one app and have empty boxes in another.

Flattening presses the stickers into the page. What you saw becomes part of the sheet, and the separate stickers are gone.

When flattening helps

Sending a filled form. Once your answers are flattened, the recipient can't accidentally change them by clicking in a box, and they look the same in every viewer. Fill the form first with Fill PDF Form, which also has a flatten option at the end, or flatten the filled file afterwards.

Comments that look different in different apps. A reviewed document with highlights and notes may show them in Acrobat but not in a phone preview, or print without them. Flattening makes them part of the page so everyone sees the same thing.

Before combining or rearranging files. Form fields don't always survive being merged with other PDFs. Flattening first keeps the filled answers as they look.

Archiving a final copy. A flattened version of a submitted form records what was submitted, without editable boxes that invite changes.

What our flatten tool keeps and changes

You can choose to flatten form fields only, comments and annotations only, or both. The tool then tells you how many it flattened. If a field can't be drawn, which is rare and mostly happens with some non-Latin text, it tells you that too.

What flattening doesn't do

This is where people get caught out.

It isn't security. The text on a flattened page can still be selected and copied, and someone with PDF editing software can still change the page. To stop people opening the file without a password, use Protect PDF.

It isn't redaction. If someone drew a black box over a number as an annotation and you flatten it, the box becomes part of the page, but the text underneath is still in the file and can often be copied out. To remove text properly, use Redact PDF, which rebuilds the marked pages as images so the hidden text is gone.

It isn't reversible. A flattened file can't be un-flattened back into fillable fields. Keep the original if you might need to edit it again.

It doesn't shrink the file much. Flattening isn't compression. If the file is too big for an upload, compress it separately.

It doesn't add a signature. If the form has a signature box, add your signature with Sign PDF after filling. A pasted signature image is not a DSC or Aadhaar eSign, and whether it's accepted is up to whoever receives the form.

Flattening and digital signatures

If a PDF already carries a digital signature, for example one applied with a DSC, any change to the file makes that signature show as invalid or changed. Flattening is a change. If you've received a digitally signed document, don't flatten it. If you're the one applying a DSC, fill and flatten first, and make the digital signature the very last step.

How to flatten a PDF

1. Open Flatten PDF and add your file. One PDF, up to 100 MB.
2. Choose what to flatten: form fields, comments and annotations, or both.
3. Check the count of what was flattened.
4. Download flattened.pdf, and keep the original.

Open the result and click a few of the former fields. They should no longer respond, while the answers stay in place.

"There's nothing to flatten"

The PDF has no form fields or annotations of the kind you chose. Text added with a PDF editor or typed into a Word document before saving is already page content. That's also true of most scanned forms.

"Print to PDF" as a shortcut

Printing a filled form to a new PDF often gives a similar result, because the printer only receives what's drawn on the page. But you can't control what's included: some apps leave out comments when printing, and a viewer that doesn't draw a field properly will print it empty. A dedicated flatten step is more predictable.

Questions people ask

Does flattening make a PDF read-only?

Not in a strict sense. It stops form fields and comments being edited as fields and comments. The page itself can still be edited with PDF software, and there's no password involved.

Will my answers in Hindi survive flattening?

In most cases fields are drawn as they look, but some non-Latin text can't be drawn by the tool, and it will tell you if that happens. Our Fill PDF Form tool doesn't flatten forms with non-Latin answers. Keep an unflattened copy and check in Acrobat Reader that the answers show.

Should I flatten before uploading to a portal?

Only if the instructions don't say otherwise. Some organisations process the form fields automatically and need them intact. If they've asked for a fillable PDF, send the unflattened copy.

Does flattening turn the pages into images?

Not with our tool. The fields and comments are drawn into the page, but the page text stays real text that you can select and search, and quality is unchanged.