A filled-in PDF form is still a form: anyone who opens it can click a box and change your answer, and some phone apps and printers show empty boxes because they don’t draw form fields at all. The same goes for comments, highlights, stamps and drawings added in a PDF reader. They sit on top of the page, can be moved or deleted one by one, and look different from one app to the next. Flatten PDF makes all of this part of the page itself, exactly as it looks now.
This is what you want before you send a finished application, an invoice you filled in, a signed contract or a marked-up assignment. The text in each field stays where it is and keeps its look, check marks and choices stay ticked, and highlights and notes are drawn into the page. Nobody can change the answers by accident any more, and every PDF reader, phone and printer shows the same thing.
Choose what to flatten. Form fields and comments together is the usual choice. Form fields only fixes the answers but keeps comments as comments, so a reviewer can still reply to them. Comments only draws in the highlights, notes and stamps while leaving the form fillable, which is useful when you annotated a form you still need to complete. Links stay clickable either way, and notes that were set to hidden are removed instead of drawn.
Everything happens in your browser on this device, and your file is never uploaded. That matters with forms, because they are usually full of personal details such as names, addresses, ID numbers and bank information. There is nothing to install and no account to create.
Flattening can’t be undone in the new file, so keep your original if you may need to change an answer later. If a form field shows text in a script the standard form font can’t write, the appearance already saved in the file is kept as it is. To add a signature before you flatten, use Sign PDF; to fill a form first, use Fill PDF forms; and to protect the finished file with a password, continue with Protect PDF.