Flatten PDF: fields, comments and signatures fixed on the page
Flattening a PDF writes onto the page itself what the file keeps apart: form fields, comments and the visible signature stop being editable and become a fixed drawing, while the text stays searchable. For a PDF that is not editable at all, each page can become an image. Everything runs in your browser.
Your PDF never leaves your device
The PDF
Everything happens in this tab; your PDF does not change, a new flattened file is created.
How to flatten
Missing: the PDF.