“Flatten the PDF” comes up most often for one of two reasons: a fillable form needs to become a permanent record once it’s filled in, or comments and markup added by reviewers need to become part of the page rather than a layer someone could hide or delete. Both are the same underlying operation.
What flattening actually does
A PDF isn’t always just static ink on a page. It can carry form fields (text boxes, checkboxes, signature lines someone can still edit), annotations (comments, highlights, sticky notes, stamps), and occasionally distinct layers that a viewer can show or hide. All of that lives as separate, addressable objects sitting on top of the page content — which is exactly what makes a fillable form fillable, and exactly why it’s a problem once you need the document to stop changing.
Flattening merges everything — the underlying page plus every field, mark, and layer — into a single, fixed layer of content. After flattening, a filled-in form field is just text on the page, indistinguishable from text that was always there. A highlight is part of the page’s appearance, not a removable object. Nothing left in the file can be toggled, edited, or filled differently.
The practical way to flatten a PDF
The most reliable way to guarantee a PDF is fully flat — no fields, no live annotations, no layers, regardless of what tool created them — is to rasterize it: turn every page into a plain image, then rebuild a PDF from those images.
Step 1: Run the filled-in or annotated PDF through PDF to Image. This renders each page exactly as it currently displays — form values, comments, highlights and all — as a flat JPG or PNG. Nothing on the page is still an editable object at this point; it’s a picture.
Step 2: Run those images back through Image to PDF to reassemble them into a single PDF, one image per page, in order.
What comes out looks identical to what you started with, but every field is now inert, every comment is now baked into the page, and there’s no way to select “hide annotations” and have the content disappear — because there’s no annotation object left to hide.
The trade-off
This is a real, if blunt, method: it guarantees flatness by making every page a picture. That means the resulting PDF has no selectable or searchable text of its own, since text and form values were rendered as pixels in step 1. If you need the flattened document to stay searchable, run it through OCR PDF afterward — that adds a text layer back on top of the flattened image without reintroducing any editable fields.
When you actually want the opposite
If your goal isn’t a permanent record but preventing casual edits while a document is shared — someone opening it and typing over your answers — locking it with a password is usually the better fit and keeps the text selectable: Lock PDF applies real AES-256 encryption, which stops the file being opened or altered without the password at all, rather than just hiding the fields’ editability.
Ready? Try the free PDF to Image tool to start flattening, then rebuild with Image to PDF — no sign-up, no watermark, and your file is never stored.