Lock filled-in form fields and comments into the page, so the PDF looks the same in every viewer and the answers can't be changed.
Runs in your browser; nothing is uploaded. Links keep working. Need to type into the form first? Use Fill PDF Form, which can flatten as it saves.
How to flatten a PDF
Choose your PDF. If the form isn't filled in yet, fill it first with Fill PDF Form (it can flatten as it saves).
Keep Also flatten comments, highlights, stamps and drawings ticked to lock those in too, or untick it to flatten only the form.
Click Flatten PDF, check the counts and download the new file.
What happens to each part of the page
Form fields (text boxes, tick boxes, drop-downs, radio buttons) are drawn with their current values and then removed, so the boxes can't be typed into any more.
Annotations with a saved appearance (comments, highlights, underlines, stamps, shapes and drawings) are drawn exactly where they appear, at the same size, and removed. Pop-up notes that belong to them go too.
Links stay clickable. Hidden annotations, which don't show on the page, are removed. An annotation without a saved appearance is left as it is, and the tool tells you how many.
Example
A one-page application with a name box, an "I agree" tick box and a yellow highlight on the terms: after flattening, the page has no form fields and no annotations. The name, the tick and the highlight are still there in the same places, now as part of the page, and the result shows 2 form fields and 1 annotation flattened. Text you typed stays selectable.
To make the file smaller for an upload form afterwards, use Compress PDF to size. To add a signature before flattening, use Sign PDF.
Frequently asked questions
What does flattening a PDF do?
It turns interactive parts of the PDF into ordinary page content. Form fields become the text and ticks you typed, and comments, highlights and stamps become part of the page. The PDF then looks the same in every viewer, and nobody can change the answers by clicking into a box.
Why flatten a filled form before sending it?
So your answers can't be changed or cleared by accident, and so they show up everywhere. Some viewers, phone apps and printers don't show form fields properly, but they always show page content.
Can a flattened PDF still be edited?
Not as a form: the boxes are gone. The page content itself can still be changed with a PDF editor, like any other page, so flattening isn't a way to make a document tamper-proof. Keep your original if you might need to change the answers.
Do links still work after flattening?
Yes. Links are kept as they are, so they still open when clicked. Only form fields and visible annotations such as comments, highlights, stamps and drawings are flattened.
Is my PDF uploaded?
No. The PDF is flattened in your browser and never leaves your device, which matters for tax forms, applications and contracts.