Redact a PDF

Why a black box is not redaction

This is the mistake that leaks documents, and it has caught courts, law firms, government departments and newspapers. It keeps happening because the incorrect method looks identical to the correct one.

A PDF stores text as characters plus instructions for drawing them. Adding a black rectangle adds one more instruction, painted on top. The words are still in the file, byte for byte. Anyone can select the “hidden” text and copy it, run a command-line extractor over it, or open the file in an editor and delete the rectangle. It takes seconds and no expertise.

Highlighting in white, drawing a filled shape in a PDF editor, and covering text with an image all have the same flaw. So does printing to PDF from some viewers.

What this tool does instead

Each page you mark is rendered to an image, your boxes are painted onto that image, and the original page is replaced with the result. The characters are not covered — the page that contained them is discarded. There is nothing underneath to recover because there is no longer an underneath.

The trade-off is real and worth stating: a redacted page becomes an image, so its text stops being selectable and searchable, and the file grows. Pages you did not mark are copied untouched and keep their text exactly as it was, so redacting one page of fifty costs you one page.

Verify the result yourself — open the redacted file and try to select text in the area you covered. There will be nothing to select.

Before you send it

• Check every page. The same name usually appears in a header, a footer and a signature block.
• Watch for partial identifiers. The last four digits of an account, an initial and a surname, or a full postcode can identify someone on their own.
• Remember metadata. Author names, and any bookmarks or attachments, live outside the page content and are not what this tool targets.
• Keep the original. Redaction is not reversible, which is the point — work from a copy.