Two PDFs sit on your screen. Both have black rectangles over the same word. One was redacted properly; the other wasn't. They look identical. The difference is what happens when you try to select the text behind the rectangle.
Visual redaction
Visual redaction is drawing a black rectangle (or any cover) on top of the content. The text and image data underneath remain in the file. The rectangle is an annotation layered on top.
When someone opens the file in any PDF reader and selects the redacted region, the underlying text copies to the clipboard. The redaction is decorative.
True redaction
True redaction replaces the underlying text and image data with the redaction marker. The original content is gone from the file. Selecting the redacted region copies nothing — the bytes that used to be there have been replaced.
Flint's redaction tool performs true redaction. Acrobat's dedicated Redact tool does too. Annotation tools (rectangle, highlighter, comments) do not.
The two-second test
After redacting, open the output PDF. Click and drag to select-all in a redacted region. Press Cmd-C / Ctrl-C. Paste into a text editor.
If nothing appears, redaction is real. If the original text appears, redaction is visual and the document is leaking. Always test before releasing.
FAQ
Does Acrobat's rectangle tool do true redaction?
No. The Comments rectangle tool draws an annotation on top. Use Acrobat's dedicated Redact tool (Tools → Redact) for true redaction.
Does Mac Preview do true redaction?
Older versions of Preview produced overlay-only redactions. Recent macOS versions have improved. Always test the output with the two-second test before relying on it.
Can I tell which type a tool uses without testing?
Often by the documentation — true-redaction tools usually flag the difference prominently. The select-and-copy test is the definitive check.
Why do tools still ship visual redaction?
Because the visual outcome looks the same and users don't always realise the difference. The fix is education and tooling that defaults to true redaction.
Decorative redaction isn't redaction. Use a tool that removes the content — and test the output.