Why real PDF redaction is more than a black box
Drawing a black rectangle on a PDF does not hide the text: it is still inside the file, can be selected and copied, and reappears when the shape is deleted. Court filings and press releases have leaked this way many times.
Quillo's redaction tool turns each page you mark into an image and paints the boxes into that image. The letters, pictures and hidden text layer under a box do not exist in the new file and cannot be recovered. Pages without boxes are kept exactly as they are.
Bank statements, ID copies, contracts and medical reports never leave your device while you do this. Redaction runs in your browser; nothing is uploaded.