Flattening a PDF, and what it costs you
Flattening turns everything interactive into ordinary page content. The document looks the same and behaves completely differently, and there is no way back.
What does flattening actually do?
It draws what you can see into the page and throws away the machinery behind it. Every form field, comment and stamp in a PDF is an object sitting on top of the page with its own appearance; flattening paints that appearance into the page content and deletes the object.
The result looks identical in any reader, which is the point. What changed is underneath: where there were fields and notes there is now the same ink as the rest of the page, indistinguishable from something printed there in the first place.
What happens to form fields?
Their values become part of the page and stop being data. Before flattening, a filled form holds one answer per named field, which is what lets a reader export the entries or a colleague change them; afterwards there are no fields, so there is nothing to export and nothing to correct.
Calculations and validation go with them. A field that added up three others was running a small script attached to the form, and flattening removes the form. Keep the fillable original whenever the same document will be filled in again.
When do you actually need it?
Whenever a document is finished and has to survive other people’s software. Print shops, court and grant submissions and anything that will be scanned back in are the usual cases: a form that renders differently in one reader, or that a recipient can still type into, is a problem flattening removes at the source.
It is also how you stop annotations travelling. Comments carry the name of whoever wrote them and the time they wrote it, so a review copy passed on without flattening takes the whole conversation with it.
What do you lose, and can you undo it?
You lose editability, and no. Fields, comment threads and layer switching are gone from the file, and no tool reconstructs them, because what they held is not stored anywhere any more. Flatten a copy and keep the original.
Two more consequences are worth knowing. A document that carried optional layers keeps whichever ones were visible and loses the switch, and a document that was already signed loses the validity of that signature, because flattening rewrites the pages it covered. Flattening is also not redaction: it fixes what is visible, and text hidden under a black rectangle is still in the file.
Where to go from here
Flatten PDF does exactly this to fields and annotations, Fill PDF form can flatten as it saves, and Edit PDF is where the notes and shapes came from in the first place.
How PDF forms work explains the fields flattening consumes, and why editing PDF text is limited covers what you can still change afterwards.