Merge the interactive parts of a PDF into the page itself. Filled form fields, annotations and signature appearances stop being separate editable objects and become fixed content. Flatten a completed form before sending it on, and the recipient sees the values permanently on the page rather than in fields they could clear or alter.
Because the values in a form field remain editable, and some readers will offer to clear or reset them. Flattening fixes the answers into the page so what you sent is what gets read.
No. The fields no longer exist as fields afterwards, so keep the unflattened original if you might need to change the answers.
Yes - flattening converts fields into ordinary page content rather than into an image, so the text remains real text.
Flatten to keep the content visible but fixed. Sanitise to remove active content altogether, including the scripts behind the form.
Files are uploaded over an encrypted connection and processed on CoonTool's own server, not in your browser. The working copy is deleted automatically once the job is done, and in any case within 24 hours. Nothing is stored permanently and no account is required.