Short answer: drop the filled form and download the flattened copy. Fields become part of the page, so a recipient can read every value but cannot change one. If the file has no form fields, the tool says so instead of returning a byte-identical file and calling it flattened.
Why send a flattened form at all
A live form invites edits: a total can be retyped, a date changed, a checkbox moved. Once flattened, the document is what it looks like. Offices that accept a “signed PDF” usually mean exactly this — a document whose values cannot move after the fact.
When it is the wrong tool
If the form must be filled in by someone else, do not flatten it: send the original. And if the page is a scan with a signature drawn on top, there are no fields to flatten — this tool will tell you there was nothing to do.