How to Redact Sensitive Information and Black Out Text in PDF Permanently

This is one of the more consequential mistakes people make with sensitive documents: drawing a black rectangle annotation over a Social Security number or account number in a basic PDF viewer feels like it hides the information, but it does nothing of the sort. A standard highlight or shape annotation sits in its own separate layer above the page content — it's visually opaque, but the actual text characters underneath are completely untouched in the PDF's data structure. Anyone who selects that area with their cursor and copies it, or opens the file in a tool that lists all text content, gets the "redacted" number back in plain text, box and all.

Real redaction has to operate at the content level, not the visual level: the actual character codes and glyph data for whatever's being hidden need to be permanently stripped out of the PDF's internal structure and replaced with the black box, not merely covered by one. Once that's done, there's no underlying text left to extract — the information isn't hidden, it's gone.