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How to redact a PDF properly (and why black boxes often fail)

Published September 28, 2026 by the ModifyPDF team

Redaction means permanently removing sensitive information from a document before it is shared. It sounds simple, yet failed redactions regularly make the news: court filings, government reports and company documents published with the “hidden” text still sitting in the file for anyone to copy. The cause is almost always the same mistake.

The black box mistake

A PDF page is built in layers. When you draw a black rectangle over a line of text with an ordinary editor or annotation tool, you add a new shape on top of the text. The text underneath is still there. Anyone can:

  • select the area and copy it, then paste it into another program;
  • use a PDF editor to move or delete the rectangle;
  • search the document for a word and find it under the box;
  • extract all the text from the file with a basic tool.

The same applies to highlighting text in black, setting the font color to white or placing an image over the words. They all change what you see, not what the file contains.

What real redaction does

True redaction deletes the underlying content in the marked area, so there is nothing left to recover. In ModifyPDF’s Redact PDF tool, pages that contain redactions are flattened into images with the marked areas blacked out. The hidden text is gone from the file rather than covered. Pages without any redactions are left untouched and keep their selectable text.

The trade-off is that the text on a redacted page is no longer selectable or searchable. For most purposes, such as sharing a statement, a contract or a report with personal details removed, that is exactly what you want.

What should you redact?

Look for anything that identifies a person or gives access to an account:

  • names, home addresses, dates of birth and signatures;
  • phone numbers and email addresses;
  • account, card, policy and ID numbers;
  • medical, salary or other personal details that aren’t needed by the recipient;
  • information about third parties who aren’t part of the conversation.

Also think about indirect clues. A job title and a small office can identify someone as reliably as a name.

Step by step

  1. Open Redact PDF and choose your file. It stays on your device.
  2. Draw boxes over anything you want to hide, or search for a word or phrase to mark every match at once.
  3. Use the one-click buttons to find email addresses, phone numbers and card numbers. Card numbers are checked with the Luhn test to avoid false matches.
  4. Review every page. Automatic detection is a help, not a guarantee: numbers written in unusual formats or split across lines can be missed.
  5. Apply the redactions and download the new file. Keep your original somewhere safe and share only the redacted copy.

Check your file before sending it

Open the redacted PDF and try to break it:

  • Press Ctrl+F (Cmd+F on a Mac) and search for a name or number you removed. It should not be found.
  • Try to select text in a redacted area. On a redacted page, nothing should highlight.
  • Copy all the text on the page and paste it into a plain text editor to see what remains.

Don’t forget hidden information

Text on the page isn’t the only place details hide. A PDF can carry the author’s name and other metadata, form field values, comments and bookmarks. If a document has comments or form fields you don’t want to share, remove them in the program that created the file before converting to PDF, or print the final version to a new PDF so only the visible pages remain.

Covering is fine for some jobs

Not every white-out needs to be permanent. If you are simply tidying a form, for example covering an old date to type a new one, Edit PDF’s white-out shape is quicker and keeps the rest of the page as text. Just remember that the original content is still underneath, so never use it for anything confidential.