What does flattening do?
It converts every filled-in field into permanent page content and removes the interactive form, so the values can no longer be edited by anyone who opens the file afterward.
Can I add new fields to a PDF that doesn't have any?
No — this tool only fills in fields that already exist in the PDF's form. Creating a new fillable form from a flat PDF isn't supported yet.
Why does it say there are no fillable fields?
The PDF doesn't have a real AcroForm — some documents look like forms visually but have no actual interactive fields embedded.
Is my PDF uploaded anywhere?
No, it's processed entirely in your browser. It's only sent anywhere when you choose to download the result.