How it works
A black rectangle drawn in a PDF reader removes nothing: the text stays in the file and comes back with copy and paste. Here you draw the areas with the mouse on each page, then the tool converts every page to an image, blacks out the areas in the image and rebuilds a PDF from those images alone: no text layer, no bookmarks, no metadata, no attachments. It then tries to extract text from the file it just produced and shows the result. The PDF is no longer searchable: that is the price of a redaction that holds.
FAQ
Why isn't the black rectangle in my PDF reader enough?
Because it is a shape placed on top of the text, not a deletion. The text stays in the file: Ctrl+A, Ctrl+C into a text editor and it reappears. That is how court filings and government documents have leaked.
Is the resulting PDF still searchable?
No: every page becomes an image. That is what guarantees nothing is left to recover. If the recipient needs the text, they can run OCR, which will only read what is visible.
How can I be sure nothing is left?
The tool reopens the file it just produced and tries to extract text, bookmarks, attachments and metadata. The report shows “0 characters” under “Recoverable text”. You can repeat the test yourself: open the PDF, select all, copy, paste.