What does it mean to flatten a PDF?
Many PDFs contain interactive layers on top of the page. A form has fields you can type into and boxes you can tick. A reviewed document has highlights and other markup that a viewer draws over the text. These items aren't part of the page itself: each one is a separate object that can be edited, moved or deleted. Flattening draws them into the page, the same way ordinary text and graphics are stored. The page looks the same, but the items stop being interactive.
Why flatten a PDF?
The most common reason is to hand over a finished form. Once flattened, the answers can't be changed by accident in the next viewer, and they print and display the same everywhere, including in apps that handle forms poorly. Flattening review markup does the same for highlights: they become a permanent part of the page, and other tools no longer list them as comments. It also helps when a system you send the file to only accepts documents without form fields.
What this tool flattens
- Form fields
- Text fields, checkboxes, radio buttons, dropdowns and list boxes. The value you see, or the tick or selection, stays on the page, and the field is removed from the form.
- Highlights and text markup
- Highlights, underlines, strike-throughs and squiggly lines.
- Drawings and shapes (optional)
- Freehand drawings, rectangles, circles, lines, stamps and text boxes. They aren't selected by default, because people often want to keep these editable.
An item is only flattened when its stored appearance, the picture the PDF keeps for it, can be drawn exactly where a viewer shows it. The tool never invents a new look for anything.
What you see before you flatten
When you drop a PDF on the tool, it first counts what it can flatten in each category and what it will keep: links, sticky-note comments, signature fields, attachments and anything else. You choose the categories, and nothing changes until you press Flatten Selected Items. If the file has nothing this tool can flatten, it tells you so and doesn't create a copy.
What it keeps
Signature fields are never flattened, and neither are sticky-note comments or links. Web links still open, and notes still open and can be read or deleted. Buttons and form fields of unusual types are kept. Embedded attachments, document scripts, the title, author and other metadata (including XMP) are kept too. Some items can't be flattened reliably: hidden fields, fields without a stored appearance, transparent markup, and markup on a layer that can be switched off. Those are left exactly as they were and listed before you flatten, so nothing disappears silently.
How the result is checked
After flattening, PDFSanitize re-opens the new copy. It first confirms that the selected items are no longer interactive. Then it checks that the kept items, the metadata, the attachments and the page count match the original. Finally, it renders every changed page, before and after, inside your browser, and compares the two images pixel by pixel. If anything doesn't match, the result says “Flattened with limitations” and lists what differs, instead of reporting success.
Flattening is not redaction
Flattening changes how items are stored, not what the file contains. Text underneath a highlight is still there and can be selected, copied or searched. Values drawn from form fields become page text that anyone can read. If you need information permanently removed, flattening won't do it: use Redact PDF, which rebuilds the redacted pages so the covered content isn't kept.
Flattening does not remove PDF metadata, attachments, JavaScript, hidden page text, or securely redact content. To remove the author, title and other properties, use the PDF Metadata Remover.
Flatten, clear or remove?
These tools do different jobs. Flattening keeps what you see and removes the interactivity. If you want to empty a form's answers instead, and keep it fillable, use Clear PDF Form Fields. If you want review markup gone rather than fixed in place, use Remove Comments From PDF for sticky notes or Remove Highlights From PDF for highlights.
Signed and archival PDFs
Flattening rebuilds the document, which invalidates any existing digital signature, so you're asked to confirm first when one is detected. For PDF/A archival files, the XMP identification is kept, but PDFSanitize doesn't validate whether the flattened copy still conforms to PDF/A. You're asked to confirm that too.
Private by design
The whole process runs in your browser. The PDF isn't uploaded, and the page blocks outgoing network connections while the tool runs. The rendering used for the visual check also happens on your device. Your original file is never changed: you download a new copy named with “-flattened” at the end. Afterwards, the PDF Privacy Checker can show what's still in the file.