A black box erases nothing: redacting a PDF properly

There is a move we have all made that does not work: open the PDF, draw a black rectangle over the sensitive bit, save, send. It looks redacted. The text is still under there, whole, and comes back the moment somebody selects it with the mouse.

From today files.co has a tool that does it properly: Redact a PDF.

Why the rectangle fails

A PDF is not a photo of a page. It is a list of instructions: put this text here, draw this line there. When you add a black rectangle you are appending one more instruction to the list. The earlier ones are not removed. The viewer paints the text, then paints your box on top — but the text is still part of the document.

Which is why selecting the area and copying it, or running the file through any text extractor, reveals what was supposed to be hidden. It has happened with court filings, official reports and contracts.

What the tool does

You mark what should not be read, and on processing, the marked page is rebuilt from scratch as an image with the boxes already baked in. The original page is gone from the file — not covered, gone.

Worth knowing before you use it:

• Marked pages lose their selectable text and get heavier. That is the price of leaving nothing recoverable.
• Pages you do not touch are copied as they were, text and all.
• The metadata of the resulting document is cleared along the way, because an Author field or a subject line can repeat precisely what you just covered.

Check before you send

The tool removes what you mark, not what you forget to mark. If the detail appears on several pages, cover it on all of them. And keep the original somewhere: there is no undo, which is exactly what makes it useful.

Without uploading anything

A document that needs censoring is not a document you want sitting on somebody’s server. The whole process happens in your browser, on your machine, offline if you like.

Open the redaction tool