To redact a PDF properly, the hidden words have to be removed from the file, not just covered. A black rectangle, a black highlight or a white box drawn on top hides text from view, but the text is still there underneath, and anyone can select it, copy it and paste it somewhere else. Use a real redaction tool, then check the result before you send it.
The black box mistake keeps happening
This isn’t a beginner’s error. It has caught out lawyers, government departments and newspapers for years:
- January 2019: lawyers for Paul Manafort filed a document in his case with black bars over sensitive passages. A Guardian reporter found the text could be copied and pasted into a new document, revealing details about his contacts with Konstantin Kilimnik (Columbia Journalism Review).
- 2014: redactions in a leaked CIA presentation published by The New York Times could be bypassed by highlighting and copying, exposing an NSA agent’s name.
- 2011: a federal judge’s opinion in Apple v. Samsung had redacted passages that could be copied into a text document.
- A 2011 study estimated that “thousands, and probably tens of thousands” of documents in the US federal courts’ PACER system had redaction failures.
The last three are from the American Bar Association’s round-up of embarrassing redaction failures.
Why covering doesn’t work
A PDF isn’t a picture of a page. It’s a list of instructions: put these words here in this font, draw this shape there, place this image. When you draw a black box, you add one more instruction, draw a black rectangle, on top. The instruction that draws the words is untouched, so:
- Copy and paste pulls the text from under the box.
- Search finds it.
- Screen readers read it aloud.
- Text extraction by any program, including search engines, sees it.
The same goes for black highlighting in Word before saving as PDF, and for white-out boxes.
What proper redaction does
Real redaction removes the content under the box from the file. In the JFDI PDF toolkit, the Redact tool works like this:
- Open the PDF in the toolkit and go to Edit & sign.
- Choose Redact (shortcut X) and drag over every name, number or passage that must go.
- Download. Each page with a redaction is rebuilt as a 200 dpi image with the black boxes burned in, so the hidden words no longer exist anywhere in the file.
The trade-off: the rest of a redacted page can no longer be selected or searched, because the page is now a picture. Pages without redactions stay as normal text. Page numbers and watermarks are added afterwards, so they stay sharp.
Nothing is uploaded at any point. The PDF is opened and rebuilt inside your browser, which is the point when the document is confidential.
Don’t forget the hidden information
Visible text isn’t the only thing that leaks. PDFs can carry:
- Document details: the author’s name, the software used, creation and editing dates.
- Editing history: some programs save changes by adding to the end of the file, so earlier versions of a page can still be inside.
- Attachments, scripts and form data.
- Text under scans: a scanned page that’s been through OCR has an invisible text layer behind the image.
In the toolkit’s Finish tab, tick Remove hidden information before downloading. It strips the author, software, dates, embedded metadata, scripts, attachments and editing history. And check the file name: “Smith-v-Jones-settlement-£2.4m.pdf” can give away what the redactions hide.
White-out or redact?
| White-out | Redact | |
|---|---|---|
| Looks like | A white (or coloured) box | A black box |
| The hidden words | Still in the file. Anyone can copy them. | Removed for good |
| Rest of the page | Unchanged | Turned into an image |
| Use it for | Tidying up, hiding a typo, making space for new text | Names, addresses, account numbers, anything confidential |
The toolkit’s Change existing text tool also covers the original words rather than removing them, so never use it to remove private information.
Check before you send: five tests
- Select all and copy (Ctrl+A, Ctrl+C) on each redacted page, then paste into a text editor. Nothing hidden should appear.
- Search the PDF for a redacted word, such as a surname.
- Open it in a different PDF reader: your browser’s built-in viewer and one other.
- Check the document properties (File › Properties in most readers) for the author and title.
- Read the file name.
Common questions
Can a redaction be undone?
Not when the content has been removed, as above. A drawn box can be “undone” by anyone with a PDF editor, by simply deleting the box.
Is redacting a scanned document different?
A scan is already a picture, so a black box burned into the image hides it. But scans often have an invisible OCR text layer; a real redaction tool removes that too, which is why the toolkit rebuilds the page.
Does Adobe Acrobat do this properly?
Yes, when you use its Redact tool and apply the redactions; drawing a black shape with its comment tools doesn’t. The same check above works for any tool.