Black boxes that don't hide anything: how to redact a PDF properly
You draw a black box over your account number, save the PDF and send it. The recipient selects the area, presses copy — and pastes your account number. This happens far more often than people think.
Why it fails
A PDF has layers. A rectangle drawn in an editor is a new shape on top; the text underneath is still there, searchable and copyable. Deleting the shape, or just copying the text, reveals it.
It has happened to the professionals
- In 2019, lawyers in a high-profile US court case filed a document whose “redacted” passages could be read simply by copying the text.
- In 2023 a flaw nicknamed aCropalypse showed that screenshots cropped with the built-in editors of Google Pixel phones and the Windows Snipping Tool could keep the cropped-out part in the file, where it could be recovered.
If it happens in courts and big tech, it happens with payslips sent to landlords.
What to hide before sharing
- ID, passport, tax and social security numbers
- Bank account and card numbers, balances and transactions that aren't needed
- Home address, personal phone and email
- Signatures, QR codes and barcodes
- Other people's names and data
Redact the safe way
- Open the redaction tool and drop the PDF or screenshot.
- Drag boxes over what to hide, or let it find emails, phone numbers and long numbers in the PDF text.
- Check every page — text split over two lines can slip through.
- Download. Each page is rebuilt as an image with the boxes painted in, so what's under them no longer exists.
Use solid boxes, not blur or pixelation: blurred text can sometimes be reconstructed.
Test it yourself
Open the result, try to select the hidden area and search for the hidden word. Nothing should come up.
If the receiver only needs proof that you are you, a watermarked copy limits misuse even further.