How to black out text in a PDF so it can’t be recovered

Handeware · updated 22 August 2026

Draw a black rectangle over a bank account number in Markup, send the file, and the number is still sitting right there in the document. Anyone who receives it can select the text underneath, copy it out, and read it. This is not a rare bug or an old iOS version — it is simply how drawing on a PDF works, and it is why redaction failures keep reaching the news.

Why the black box doesn’t work

A PDF is actually holding two separate things at once: instructions for drawing the page, and the text itself, stored as real characters. When you draw a shape in Markup, Preview, or almost any annotation app, all you have done is add one more drawing instruction that says “fill this rectangle with black.” You have put something on top of the text. You have not touched the text.

So the characters are still there, in order, exactly where they always were. Any of the following gets them back in seconds:

• Open the PDF and drag-select across the black box, then copy.
• Open it in a browser and use Find to search for the digits.
• Run any text extraction tool over the file.
• Delete the annotation layer — often just a single click.

A rectangle like that is a sticker, not an eraser. It has caught out law firms, government departments and newspapers, all of whom had professional tools and used them the wrong way.

How to check a file you already sent

Do this before you trust any redaction, including the ones you did months ago:

1. Open the PDF on a Mac or PC.
2. Press Cmd + F (or Ctrl + F) and search for something you believe you hid — a few digits of the account number, a surname, a street.
3. If it finds a match, the redaction did not work, and the file is still exposed wherever you sent it.

You can also just drag across the blacked-out area and paste into any text box. If characters show up, they were never actually removed.

What actually removes the text

Real redaction has to destroy the characters, not cover them up. There are three approaches that genuinely work, and I have put them in increasing order of convenience.

1. Flatten the page to an image

If a page becomes a picture, there is no text left to recover. On an iPhone that means screenshotting the page, marking up the screenshot, and sharing that instead. It works, and it is clumsy: you lose page size, quality and searchability, and it is easy to miss a page in a long document.

2. Print to PDF after annotating — carefully

Printing to PDF sometimes rasterises the annotation into the page, and sometimes leaves the text layer sitting underneath it, depending on the app and the OS version. Because the outcome varies, you have to verify it with the Find test above, every single time. A method you cannot predict is not really a method.

3. Use a tool that burns the redaction in

The reliable answer is software that removes the underlying content instead of painting over it. On a phone, that means an app that re-renders the page with the hidden region genuinely gone before it ever writes the file out.

Doing it on an iPhone

Chartra is the document scanner I built, and its Hide tool exists for exactly this problem. You drag over the area you want gone and it is burned into the page — the words underneath are removed, not covered, so there is nothing left to select or search for. Everything happens on the phone; the document is never uploaded, which matters when the thing you are hiding is the reason you care.

Whatever you use for this, including mine, run the Find test on the exported file before you send it anywhere. A redaction you have not verified is really just a redaction you are guessing about.

Common questions

Does Markup’s highlighter work instead?

No. Every tool in Markup draws on top of the page. Pen, shape, highlighter, text box — all of them leave the original characters exactly where they were.

What about a black box in Pages or Word before exporting?

Same problem, just one step earlier. The shape sits above the text in the document and usually survives the export intact. Test the exported file the same way.

Are scanned documents safe automatically?

Not necessarily. Most scanner apps add an invisible, searchable text layer on top of the image — that is the whole reason a scan can be searched at all. If that layer exists, a drawn box only hides the picture of the words, while the recognised text sits behind it, untouched. The same test applies here too.

Can I undo a bad redaction I already sent?

No. Once the file has left your hands, assume the hidden content is already known, and act accordingly — change the password, notify the person, reissue the document. Redo the redaction properly before you send a replacement.