Blacking out text in a PDF seems straightforward — just draw a black rectangle over the sensitive content, right? Unfortunately, this approach is one of the most common and dangerous mistakes people make with document security. A black box drawn over text in a PDF is nothing more than a visual annotation. The actual text data remains fully intact in the file, and anyone with basic knowledge can extract it in seconds.
This has led to real-world data breaches. Government agencies, law firms, and corporations have inadvertently exposed classified information, personal data, and confidential details because they used visual overlays instead of true redaction. In one well-known case, a U.S. government filing had names blacked out with rectangles, but journalists simply copied and pasted the "hidden" text to reveal the identities.
The Critical Difference: Covering vs. Redacting
To understand why black boxes fail, you need to know how PDFs work. A PDF file contains multiple layers of data: the visual rendering layer (what you see on screen), the text layer (searchable and selectable text), and the metadata layer (author, creation date, and other properties). When you draw a black rectangle using an annotation or shape tool, you are only adding an element to the visual layer. The text layer remains completely untouched.
Here is what happens when you "black out" text with the wrong tools:
- Copy and paste still works. A user can select the area behind the black box and paste the text into another application.
- Search finds the hidden text. Using Ctrl+F or Cmd+F reveals the text beneath the overlay.
- Annotation removal exposes everything. PDF editors can delete the black rectangle with a single click, revealing the original text.
- Text extraction tools ignore overlays. Command-line tools and libraries can extract all text from a PDF regardless of visual layers.
How to Properly Black Out Text (True Redaction)
True redaction uses specialized tools that modify the PDF at the data level, permanently removing the text from both the visual and text layers. After proper redaction, the content does not exist in any form within the file.
Option 1: AI-Redact (Free, Recommended)
AI-Redact is a browser-based tool that performs true permanent redaction. Upload your PDF, and the AI automatically detects sensitive information including names, Social Security numbers, email addresses, phone numbers, and financial data. You can also manually select additional areas to redact. Once applied, the text is permanently removed from the document. The free tier supports PDFs up to 4 pages, and no signup is required.
Option 2: Adobe Acrobat Pro ($239.88/year)
Adobe Acrobat Pro includes a dedicated Redact tool (Tools > Redact > Mark for Redaction). This is the only tool in Adobe's suite that performs true redaction. Do not confuse it with the highlighter, comment tools, or drawing tools, which are commonly but incorrectly used for blacking out text. The downside is the high annual cost and the lack of automatic PII detection.
Option 3: Print, Black Out, and Rescan (Last Resort)
As an absolute last resort, you can print the PDF, physically black out text with a marker, and scan it back in. This eliminates the digital text layer entirely because the scanned version is essentially an image. However, this approach destroys document quality, eliminates text searchability, increases file size, and is impractical for anything beyond a single page.
How to Verify Your Redaction Worked
After blacking out text, always verify that the redaction is truly permanent:
- Try to select the text. Click and drag over the blacked-out area. If you can select text, the redaction is not permanent.
- Use the search function. Press Ctrl+F (or Cmd+F on Mac) and search for a word you know was in the redacted area. If it appears in results, the text was not removed.
- Open in a text editor. Open the PDF in a plain text editor. While most content will be encoded, if you can find readable strings of the "redacted" text, the redaction failed.
Tools That Do NOT Provide True Redaction
Be aware that the following tools and methods create visual-only overlays, not true redaction:
- Adobe Reader's highlight and drawing tools
- Mac Preview's markup and annotation features
- Microsoft Word's shape/rectangle tools
- Any image editor used on a PDF screenshot
- Google Docs comment or suggestion features
- Most free "PDF editor" apps that only add annotations
If you are handling sensitive data — whether for legal, medical, financial, or personal reasons — always use a dedicated redaction tool that permanently removes content from the PDF data structure.