Why the text survives the box
A PDF page is a list of drawing instructions. Text is one kind: put this character at these coordinates, in this font, at this size. A rectangle is another kind: fill this area with this color. Painted after the text, the rectangle hides it.
Hiding and removing are different operations, and the file only performed the first one. The text runs are still in the instruction list, in the order they were written.
The clipboard doesn’t read pixels. Select-all walks the text runs, so whatever is still in that list comes out, black box or no black box.
Four ways a black box gets made
These are the four things people mean by “redacted”. Only the last one removes anything.
- A shape or freehand annotation drawn over the text. By far the most common, and the one that leaks: selection, search and copy all still find the covered words.
- A filled rectangle plus the text recolored to match it. Identical on screen. The characters are still in the content stream and still extractable.
- The characters deleted from the content stream. The words are gone. The layout usually isn’t fine afterwards, because a gap opens where the line was, which is why so few tools do it.
- The page redrawn as an image. Nothing is left to extract, because there’s no longer any text, only a picture of the page. The Redact tool on this site does this page by page, and only for the pages you box.
Ten seconds to check any redacted PDF
Do this before trusting a file, whoever sent it and however official it looks.
- Open it in any reader and press Ctrl+A, then Ctrl+C.
- Paste into a plain text editor. Not a word processor. You want the raw characters, not formatting.
- Search for the digits, the name, the string you were told had been removed.
- If it comes back, the sender covered the text in a picture. The file still contains it, and so does every copy of the file.
Why this comes up so often
A redacted document is usually going somewhere it shouldn’t be readable: a landlord, a contractor, a support inbox, a public filing. The box is the reassurance. The text underneath is the state of the file, and it travels exactly as far as the box does.