Why black bars in PDFs don't work (with example)

Two seconds of copy-paste makes the "redacted" text readable again. Here's the technical explanation with a concrete example — and what to do instead.

PDFs use a layer model. Text and images are separate objects, stacked on top of one another. When you draw a rectangle over text, you're simply adding a new object on top — the text underneath remains completely intact.

The demonstration

1. Open a PDF where someone has used "black bars" to hide text.
2. Select the entire document (Ctrl+A).
3. Copy to clipboard (Ctrl+C).
4. Paste into a text editor.
5. All "redacted" text is readable.

Variations that also don't work

• White text on white: same problem — the text is still there.
• Opaque image placed over text: copy-paste retrieves the text hidden beneath the image.
• Snipping tool screenshot + black box: the screenshot itself is fine, but when you export it to PDF it loses precision, and metadata is still retained.
• Text highlighting with a black background colour: that's a text highlight, not redaction.

Historical incidents

• Various courts published "redacted" judgements between 2010 and 2020 that could be read again within seconds.
• A TNO report with redacted names — the same pattern.
• WikiLeaks / Panama Papers — examples where redaction errors led to leaks.

What actually works

Proper redaction software removes the underlying text objects from the PDF entirely, rather than merely covering them up. See the redaction guide for the full process.

See also: stripping metadata, audit trail.