The problem this solves
You download a form, fill it in, save it, email it. The person at the other end replies that it arrived empty. You open your copy and it is perfectly filled in.
Neither of you is doing anything wrong. A filled PDF form is genuinely two things stacked on top of each other: the printed page underneath, and a set of interactive fields above it holding the values you typed.
Whether the values appear depends entirely on what the reader is doing. Full PDF software renders the fields. Plenty of other things do not, and there are a lot of them: browser preview panes, the preview in a mail client, phone mail apps, older software, and some printers.
Flattening removes the distinction. The values are drawn into the page, the fields are deleted, and there is only one layer left. Everything that can open a PDF at all will show your answers, because they are now simply part of the page.
How this tool works
A filled PDF form is two things at once: the printed page, and a set of form fields sitting on top of it holding the values you typed. Flattening draws those values permanently into the page and removes the fields.
The tool counts the fields first and tells you how many it found, because a document with none has nothing to flatten and the honest answer is to say so rather than hand back an identical file.
It cannot be undone. That is the point of it, and it is why the original is left alone and the result comes back as a new file.
The other reason people do it
A form with live fields can be edited by whoever receives it. Sometimes that is fine. Frequently it is not: a signed declaration, a completed timesheet, a quotation with prices on it, a permit, or anything that becomes a record the moment it is sent.
Once flattened, the answers cannot be altered by clicking on them. They are printing on a page. Somebody could still edit the document with the right software, in the same way they could alter any PDF, but the trivial route is closed and the document is fixed for ordinary purposes.
What flattening is not
It is not making the document uneditable, and it is not turning it into an image.
The page content stays exactly as it was, including your answers, as real text. It can still be searched, selected, copied and read aloud by a screen reader, which is a good thing and worth keeping. Anyone who tells you flattening secures a document is overselling it.
If what you actually want is a document nobody can extract text or images from, that means redrawing every page as a picture, which destroys the text layer permanently and makes the file considerably larger. Compress PDF will do that if you choose to redraw the pages, and it is honest about the cost.
It checks before it does anything
The tool counts the fillable fields first and tells you what it found.
That matters because on a document with no form fields, flattening does nothing whatsoever, and a button that appears to work while achieving nothing is worse than useless. If your document has no fields you get told, along with what you probably wanted instead.
When there are fields, you get the count before you commit. If a form you filled in reports zero, that is worth knowing on its own: it means what you were typing into was not a real form at all.
Keep the original
Flattening cannot be undone, so your original is never touched. The tool reads it and writes a new file alongside.
Keep the version with live fields. If the form comes back for a correction, or you need to submit an updated one next quarter, editing the original takes a minute and re-typing a flattened one takes considerably longer.
Check it before you send it
Open the flattened copy and read it. Every answer should be visible, in the right place, in a font you can read.
It is a two second check and it is the whole reason you flattened it in the first place: so that what you can see is what they will see.
A filled form is full of your details by definition
No account, no email address, no size limit and no watermark. A filled form is full of your details by definition, and this one is opened, flattened and saved entirely by your own browser.
Common questions
I filled in a PDF form and it arrived blank. Why?
Because a filled form is two separate things: the page, and a set of fields sitting on top holding your answers. Plenty of readers show the page and not the fields, particularly browser previews, phone mail apps and older software. You see your answers, they see an empty form. Flattening draws the answers into the page so everybody sees the same document.
Can the answers still be changed after flattening?
No. The fields are gone and the values are part of the page, the same as any other printing on it. That is usually the other half of why people want this: sending a completed form that arrives exactly as it left.
Can I undo it?
No. Which is why your original is never modified. The tool reads it and writes a new file, so if something needs changing you go back to the original, change it, and flatten again.
Will the text still be searchable?
Yes. Flattening is not the same as turning the page into a picture. The page content, including the values that were in the fields, stays as real text, so it can still be searched, selected and read aloud.
My PDF has no form fields. What does flattening do?
Nothing at all, and the tool says so rather than giving you a button that appears to work. If what you want is to stop somebody editing the text or lifting the images out, that is a different job. Redrawing every page as a picture does it, and Compress PDF will do exactly that if you choose to redraw the pages.
Is my form uploaded?
No. It is opened, flattened and saved by your own browser. That matters here more than on most tools, because a filled form is by definition full of your details.