How do I black out information in a document properly?

Why a black rectangle is not redaction, three methods that genuinely remove the information, and the check that proves it worked.

Difficulty: beginner
Time: 20 min
Read: 3 min
Safety: caution

Short answer

The safest household method is to print the page, black it out with a marker, and scan the paper copy — the information genuinely does not exist in the new file. If working on screen, use a tool with a real redaction function, then verify by trying to select and copy the area.

Every few months a public body discloses something because a black box was drawn over text rather than removing it. The same mistake happens with bank statements sent to landlords and payslips sent to lenders.

Safety

Drawing a black box over text in a PDF or image editor does not remove the text underneath. It can be selected, copied or recovered, and this has caused real disclosures of addresses, bank details and medical information. Always verify by selecting and copying the redacted area before sending.

Step by step

Understand what a PDF actually contains.Text in a PDF is text, in layers. Drawing a shape over it adds a shape on top; the words remain underneath and can be selected, copied, or revealed by removing the shape.
Decide what genuinely needs removing.Account numbers, sort codes, full addresses, dates of birth, national insurance numbers, signatures, and other people's details. Also check headers, footers and page numbers.
Use the print-and-scan method for certainty.Print, black out with a thick permanent marker, then scan or photograph the paper. The digital file that results has never contained the removed text. Hold the page to the light to check the marker covers it.
Use a real redaction tool if working on screen.Some PDF software includes a redaction function that deletes the underlying content rather than covering it. Look for the word redact specifically; annotation and drawing tools do not do this.
Or flatten the document to an image.Exporting the page as an image, editing the image, and rebuilding the PDF removes the text layer. Slower and larger, but reliable, and it also removes the ability to search the document.
Verify by selecting and copying.Open the finished file, try to select the redacted area, and paste it into a text editor. If anything appears, the redaction failed. Do this every time.
Check the document properties.Files carry author names, editing history and sometimes previous versions. Use the document's inspect or remove-personal-information function before sending.
Check photographs and screenshots too.Images carry location and device data. Cropping an image usually removes the cropped area properly, but blurring can sometimes be reversed — a solid block is safer than a blur.

Tips

For bank statements requested by a landlord or lender, ask what they actually need. Often it is the balance and the name, not the transaction list.
Pixelating or blurring is weaker than a solid block. Reversal techniques exist for both, especially with text and numbers.
If you are asked to send a photograph of a passport or driving licence, add a visible watermark across it saying what it is for and the date. It does not stop copying but it makes reuse harder.

Common mistakes

Drawing a black rectangle in a PDF reader — It is an annotation on top of the text. Selecting and copying, or opening the file in another program, reveals everything underneath.
Highlighting text in black in a word processor — The text is still there and is revealed by changing the highlight, opening it elsewhere or converting the file.

If it doesn't work

Copied text still appears from behind the box

Cause: The box is an annotation, not a redaction — Fix: Start again with print-and-scan, a real redaction tool, or by flattening the page to an image.

The document is a scan, so there is no text layer

Cause: Scans are images, but many scanners add a searchable text layer — Fix: Check by trying to select text. If it selects, there is a text layer and covering it is not enough.

Already sent an incorrectly redacted file

Cause: The check was not done — Fix: Tell the recipient immediately and ask them to delete it. If it contained bank details, contact the bank; if it contained someone else's personal data, consider whether it is reportable.

Questions people ask

Is print-and-scan really the best method?

It is the most reliably safe method for anyone without professional redaction software, because it makes the removal physical. It is slower and produces a larger, unsearchable file, which is a fair trade.

Does exporting to PDF from a word processor remove hidden text?

No. Tracked changes, comments and hidden text can survive the export. Accept or remove all changes and run the document inspector first.