How to Redact a PDF Properly: Why a Black Box Often Hides Nothing

You need to send a document, but not all of it. A bank statement without the account number, a medical report without the patient's date of birth, a court exhibit without a minor's name. The instinct is to draw a black rectangle over the sensitive part and save. On screen, it looks perfect. In the file, the text is very often still there, one click away. This guide explains why that happens, how to check any redacted PDF in under a minute, and how to redact so the information is really gone.

Why a black box usually hides nothing

A PDF page is not a flat picture. It's a stack of layers: text stored as actual characters with positions, images, vector shapes, and on top of that, annotations such as comments, highlights and drawn rectangles. When you draw a black box in an ordinary PDF editor or viewer, most of the time you're adding a new shape or annotation on top of the page. The text underneath isn't touched. It's just covered, the way a sticky note covers a word on paper, except that in a PDF anyone can lift the sticky note.

The same thing happens upstream. If you black out text in a word processor by shading it or placing a filled shape over it, then export to PDF, the export usually keeps the text and the shape as separate objects. The PDF looks redacted and isn't.

Here is what a recipient can do with such a file, without any special skill:

• Select and copy. Drag the cursor across the black area, copy, paste into any text editor. The hidden words appear.
• Search. Use the viewer's search function to look for a name or number you thought you removed. The viewer finds it and highlights a spot under the black box.
• Move or delete the box. In many editors, the rectangle is an annotation that can be selected and deleted, revealing the text as it was.
• Extract the text. Any tool that converts PDF to text or reads it aloud will read the "hidden" passage along with the rest.

Failed redactions of this kind have surfaced over the years in court filings and official reports, usually discovered by someone who simply copied and pasted. It's not a sophisticated attack. It's the default behavior of the file.

A one-minute test for any redacted PDF

Before you send a redacted document, or when you receive one and want to know whether it's safe to pass along, run these checks on the final file, not on your working copy:

1. Search for a word you redacted. If the search finds it, the text is still in the file.
2. Select all text on the page and paste it into a plain text editor. Read the result where the black boxes were.
3. Try to click on a black box and move or delete it.
4. Check every page, including headers, footers and the last page, where account numbers and names tend to repeat.
5. Look at the file name. A perfectly redacted file named Statement - Martin Dubois - account 4417.pdf undoes the work.

If any of the first three reveals the information, the file isn't redacted. Start again from the original.

What real redaction means

Proper redaction removes the information from the file rather than covering it. There are two broad ways to get there:

• Remove the underlying content. Specialized redaction tools delete the text characters and image pixels under the marked area, then burn in a black box where they were. Done well, it keeps the rest of the page as real, searchable text. Done carelessly, fragments can survive.
• Rebuild the page as an image. Render the page to a picture, paint the black areas into the pixels, and build a new PDF from those pictures. Nothing from the original text layer can survive, because there is no text layer anymore. The trade-off is that the page is no longer searchable or selectable.

PDF Protect Plus uses the second approach, deliberately, because it's the one where "the text is really gone" doesn't depend on getting every detail right.

How redaction works in PDF Protect Plus

Redaction is part of the client dossier builder in the desktop app. When you prepare a dossier, the steps are:

1. Open the document's pages. In the dossier, choose Edit pages / redact for the document that contains the information.
2. Draw the masks. Choose Redact an area, then draw a rectangle over the sensitive information with the mouse. Cover it generously, with a margin around the text. Repeat for each item on each page. If a rectangle isn't right, Remove last redaction takes it back.
3. Export. When the dossier is exported, every document that has at least one mask is rebuilt entirely from images: each page is rendered at 2400 pixels, the black areas are painted into the image, and a new PDF is assembled from those images. The text that was under the boxes no longer exists anywhere in that document.
4. Check the result. Open the exported file and reread every masked area. Then run the one-minute test above; the search and copy checks should come back empty on the rebuilt pages.

Masks are drawn by hand. The app doesn't search the document for names or numbers to redact automatically, so the thoroughness is yours: go through each page, including headers and footers.

What to expect from a redacted document

Because the document is rebuilt from images, some things don't carry over, and it's better to know them upfront:

• Text in that document is no longer selectable or searchable.
• Links, annotations and comments, form fields and file attachments are not kept in that document.
• The rebuild applies only to documents with at least one mask. Other documents in the same dossier are unaffected.
• A document can have up to 200 pages for this process.
• Your original files are never modified. Keep them somewhere secure, since they still contain everything.

For most redaction needs, such as sending a statement, an exhibit or a form to a third party, losing selectable text is exactly what you want.

Then protect the result

Redaction controls what's in the file; a password controls who can open it. For a document that still contains confidential information after redaction, do both. Once the redacted dossier is exported, protect it with a password (AES-256) in the same app, and send the password by a separate channel, as described in our guide to sending a PDF password separately. If you're sending many redacted files to many recipients, the batch protection tutorial shows how to give each one its own client's password.

A redaction checklist

• List what must go before you start: identification numbers, account numbers, dates of birth, addresses, signatures, names of third parties or minors.
• Check every page, including headers, footers, page margins and stamps.
• Draw masks with a margin; a sliver of a character can be enough to guess a digit.
• Reread every masked area in the exported file, then search and copy-paste to confirm.
• Rename the file if its name contains what you redacted.
• Consider hidden data too: the article on removing metadata from a PDF covers author names and other properties that ride along with a file.

Everything described here, from drawing masks to rebuilding pages and encrypting, happens on your own computer. The documents you're redacting are, by definition, the ones that shouldn't be uploaded anywhere. For more on what stays local, see how local processing works, or download the app and try it on a test document during the trial.