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How to redact a PDF properly (and why black boxes are not enough)

Drawing a black rectangle over text hides it on screen but leaves it in the file. Here is what real redaction removes and how to check your output.

PDF100 team6 min read

Redact PDFTry it while you read: Permanently black out words, emails or phone numbers. Redacted text is truly removed.Open tool

Every few months a court filing or government report makes the news because someone "redacted" it with black highlighter in a PDF editor, and a reader recovered the hidden text by selecting it and pressing copy. The black box was only paint on top. The words underneath were still in the file.

Covering is not removing

A PDF page is a list of drawing instructions: place this text here, draw this image there. Adding a black rectangle just appends one more instruction. Every viewer still has the original text instructions, which is why copy, search and text extraction all reveal what you meant to hide.

Text on the page is not the only place sensitive information hides. It can also sit in:

  • Annotations and comments, including ones that are hidden or collapsed
  • Form field values, which live separately from the visible page
  • Document metadata such as title, author, subject and keywords
  • Bookmarks and outline titles

What proper redaction does

  1. Find every occurrence of the sensitive text, across all pages.
  2. Make the matching characters unrecoverable, not merely hidden.
  3. Delete annotations, form fields, metadata, bookmarks and attachments that could carry the text.
  4. Show an opaque box where the text used to be, so readers can see something was removed.

PDF100's Redact PDF tool finds every match, then rebuilds each affected page as an image with the black boxes burned into the pixels. The original text layer for those pages is gone, so there is nothing underneath to copy. It also deletes comments and form fields containing the text, and strips metadata, bookmarks and attachments. Because it runs in your browser, the document you are redacting is never uploaded.

The trade-off: redacted pages are no longer selectable text. Pages without any matches are left untouched. If you need the redacted pages searchable again, run OCR PDF on the output; the removed words will not come back, because they are no longer in the image.

How to check your redacted file

Never trust a redaction you have not tested. Three quick checks catch almost every failure:

  • Open the output and search (Ctrl+F) for the redacted words. You should get no results.
  • Select all text on the page and paste it into a plain text editor. The redacted words must be absent.
  • Run the file through PDF to Text and search the extracted text.

Tools mentioned in this article

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