The black box problem
The most common way people black out text in a PDF is to draw a filled black rectangle over it, or to use a black highlighter, in whatever PDF viewer or word processor they have. On screen and on paper it looks perfect. But the words are still in the file, sitting underneath the rectangle. Select the area and copy it, and the "hidden" text pastes into an email in full. Search the PDF and the hidden words are found. Delete the rectangle in any PDF editor and the page looks as it did before.
This has gone wrong in public many times. In January 2019, a court filing by Paul Manafort's lawyers had black bars over passages that reporters revealed simply by copying the text out. Similar failures have exposed names in government reports and corporate disclosures. In each case the text was covered, not removed.
The same problem in disguise
- Black highlighter in Word, then "Save as PDF". The highlight is a background colour behind the text. The text is still there.
- Black font colour on black. Still text. Still copyable.
- A shape placed over a scan. The scan underneath is complete. Move or delete the shape and the image is back.
- Printing to PDF. This sometimes flattens the page and sometimes keeps the text layer, depending on the printer driver. Never rely on it.
How to black out text properly
To black out text so it cannot come back, the characters have to leave the file. RedactKit does this by rebuilding every page you redact as an image, with the black bars painted into the pixels. The original page, with its text, fonts and drawing instructions, is left out of the new PDF entirely.
- Open your PDF in the tool on this page.
- Type the words to black out, or switch on a pattern such as email addresses or phone numbers.
- Drag across anything else: a signature, a photo, a whole paragraph.
- Press Preview to see the black bars as they will be exported.
- Export. The proof check then reads the new file and reports how much text is left on the redacted pages: none.
Black out text in your own PDF now: it runs in this tab and the file never leaves your device. Documents up to 5 pages are free. Start redacting →
How to check any blacked-out PDF
Whoever made it, you can test a redacted PDF in under a minute:
- Open it in a PDF viewer and press Ctrl+A (Cmd+A on a Mac) on a redacted page, then copy and paste into a plain text editor. If the hidden words appear, it wasn't redacted.
- Use the viewer's search for one of the redacted words.
- Look at the document properties. The author and title fields often name the person the redaction was meant to protect.
A page RedactKit has redacted is a single image, so step 1 returns nothing at all for that page, and step 2 finds nothing on it.
What black bars can't fix
Redaction removes what you mark. It can't know what you didn't mark. Common misses are the same name spelt differently, initials, an email address that contains the name, a job title that only one person holds, and the length of a black bar itself (a short bar after "Dear" is a short name). Use the word list to catch variations, and read the preview before you export.
Related: How to redact a PDF · Redact a PDF free · Remove personal information from a PDF