Why the black box over your PDF does not delete the text

It is the most common mistake made when preparing a document for release: covering the confidential part with a black rectangle. The data is still there, and whoever receives the file can recover it with no technical knowledge at all.

Why the rectangle does nothing

A PDF stores its text separately from what you see on screen. Drawing a black rectangle over a name, an account number or an address only adds a shape in front. The original text is still stored in the file, underneath. So:

• Select across the rectangle with the mouse, copy, and the hidden text appears.
• Open the PDF in a different program, or convert it, and the data comes back.
• Move or delete the rectangle and the data is simply on display again.

Check it yourself: open a PDF where you covered something with a rectangle, select across it, and paste into a text editor. If the words come out, they were never deleted.

This is not theoretical

On 8 January 2019, lawyers for Paul Manafort filed a court document with passages blacked out. The text underneath was still live: reporters copied it straight out, it was being quoted in news reports the same day, and a corrected filing had to be entered on the public docket before the day was over. Regulators fine for the same class of mistake. In October 2024 the UK's Information Commissioner's Office fined the Police Service of Northern Ireland £750,000 after a hidden worksheet inside a spreadsheet released under freedom of information exposed the surnames, initials, ranks and roles of all 9,483 of its officers and staff.

What to do instead

For the data to be gone it has to be removed from the contents of the file, not covered. And if the data sits inside a scanned image, the pixels of that area have to be cleared — not patched over.

Doing that by hand with professional PDF software is possible but slow, and the step people skip is the one that matters: checking the finished file afterwards. A redaction you did not verify is a redaction you are only assuming worked.

How TachadoPDF does it

TachadoPDF removes the text from the file and clears the pixels of the areas you mark, strips the metadata, and then reopens the PDF it just produced and searches it again — including the metadata — for everything you redacted. If any of it comes back, the download is blocked and you are told which page. You also get a verification report with the SHA-256 fingerprint of the file you deliver. Everything runs in your browser; the document is never uploaded.