How to redact a PDF properly, so it can’t be undone

Why a black box isn’t enough

A PDF keeps text, pictures and drawings in separate layers. Drawing a black rectangle in most editors, or highlighting text in black, adds a shape on top: the words are still underneath. Anyone can select them, copy them, search for them or delete the rectangle.

This has gone wrong publicly for years: court filings, government reports and company documents released with "redacted" names that a copy and paste revealed. It happens because the file looked right on screen.

How proper redaction works

A real redaction does three things:

• Removes what’s under the box. Either the text is deleted from the page’s contents, or the page is rebuilt as a picture with the box burned in, so there is no text left at all.
• Removes hidden data. The author, the software, comments, form fields, attachments, scripts and earlier versions can all hold the same information.
• Gets checked. The saved file is searched for each redacted word before it’s sent.

Redact rebuilds each page you put a box on as a picture at 200 dpi (sharp for print), with the boxes burned in. Pages without boxes are copied as they are, so their text can still be selected and searched. The new file carries none of the original’s hidden data. Then it opens the saved file again and searches its text, its raw bytes, and what each rebuilt page still shows outside your boxes, for every word that was under one.

Redact a PDF, step by step

1. Open the PDF in Redact. It stays on your device.
2. Find what to hide. Tap Find and type names, addresses or reference numbers, one per line, or tick the personal details to look for: email addresses, phone numbers, card numbers, IBANs, sort codes and account numbers, National Insurance and Social Security numbers.
3. Review every match and box the ones to remove. Patterns can miss a detail written an unusual way, so read the pages too.
4. Add anything else by hand: drag a box over it, or tap a word.
5. Redact and save. Read the check: it says whether anything you redacted can still be found, and where.
6. Open the saved file once before you send it.

The same name in another place

The most common slip is boxing a name at the top of a page and missing it in the small print at the bottom, or on page 4. Use Find with the name rather than boxing it by sight, and let the check catch the rest: Redact lists any redacted word that is still readable anywhere in the file.

Screenshots and photos

Redact works on photos and screenshots too. Solid boxes are the safe choice for text: blurred or lightly pixelated words can sometimes be guessed back. Strong pixel blocks are fine for faces and number plates. Saving a photo again from its pixels also removes its location and camera details; to see what a photo holds first, use Photo info.

When special rules apply

Some documents have their own rules for redaction: court filings, freedom of information responses (which often need a reason code on each box, such as “(b)(6)”), medical records. Redact Pro adds reason labels and a log page that lists how many items were removed from each page without recording what they were.