"Flatten" and "sign" both come up around finalizing a PDF, and it's easy to assume they're two names for the same action, since both make a document feel "done." They're not related mechanically. Flattening is about visual permanence: turning editable, interactive content into static page content. Signing is about identity and integrity: proving who approved a document and whether it's been changed since.
What flattening actually does
A PDF form has fillable fields, checkboxes, dropdowns, text boxes, that live as separate interactive objects layered over the page. Flattening merges those field values permanently into the page's static content and removes the underlying interactive form layer, so what you see becomes what's actually there, no more editable fields, no more ability to click into a box and change an answer. It's the same underlying operation whether you're flattening a filled-in form with Fill PDF Form, locking in a watermark added with Watermark PDF, or finalizing any layered edit with Flatten PDF so it can't be selectively removed later.
Flattening has nothing to do with proving who filled out the form or whether the file was edited afterward at the byte level, someone could still open a flattened PDF, screenshot a section, and paste in a different value using image editing tools. It only removes the built-in, one-click editability that interactive form fields provide.
What signing actually does
A digital signature via Sign PDF does something structurally different: it applies a cryptographic hash of the document's content at the moment of signing, tied to a certificate that identifies the signer. Any viewer that checks the signature can confirm two things: that the signature genuinely came from that identity, and that the document's bytes haven't changed since the signature was applied. If even one character changes after signing, the signature shows as invalid or broken.
This is why a signed PDF is trusted in contexts a flattened-but-unsigned PDF isn't, a flattened form proves nothing about authorship or tampering, it just prevents casual re-editing through the form UI. A signature is the part that actually gives a document legal and evidentiary weight.
Using both together
The common real-world sequence is: fill out a form, flatten it so the entered values can't be casually edited by opening the interactive fields again, and then digitally sign the flattened result so any tampering after that point is detectable. Doing it in that order matters, signing first and then flattening afterward would invalidate the signature, since flattening changes the document's underlying content after the signature was calculated.