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

Covering is not removing

A PDF page is built in layers. When you draw a black rectangle in most PDF editors, or highlight text in black, you add a shape on top of the text; the words underneath remain in the file. Anyone can select and copy them, search for them, or delete the rectangle and read everything. Court filings, government reports and corporate documents have been exposed exactly this way.

Proper redaction removes the underlying content, so there is nothing left to reveal.

How secure redaction works

There are two sound approaches. Professional editors such as Adobe Acrobat Pro have a dedicated redaction mode that deletes the text and image data under the marked area and then applies the box. The other approach, which Fileixa uses, turns each marked page into an image with the black boxes burned into the pixels and rebuilds the page from that image. Because the page no longer contains any text objects at all, nothing can be copied out from under the box.

The trade-off of the image approach is that redacted pages lose selectable text. Pages you do not mark stay as they are.

The places people forget

Most redaction failures are not technical; they are things nobody thought to check.

• Headers, footers and page margins that repeat a name or case number on every page.
• Document metadata: the Author, Title and Subject fields often contain names. Check them with a metadata tool.
• Comments, form fields and attachments embedded in the PDF.
• Images and scanned signatures, which contain no text but can still identify someone.
• Earlier versions: if you redact a copy, make sure the unredacted original is not what gets sent.

Test it before you send it

Open the redacted file, press Ctrl+F (Cmd+F on Mac) and search for a word you removed. Then select all the text on the page and paste it into a plain text editor. If either finds the hidden words, the redaction did not work.