Documents released as redacted have repeatedly turned out to hold the hidden text beneath the black boxes, because someone drew a rectangle over the words instead of removing them. The text was still in the file, and anyone could select it, copy it or find it with search. If a PDF leaves your control, treat redaction as a deletion, not a decoration.
Why a black box does not work
A shape drawn on a page is a separate object on top of the text. The text underneath is unchanged. Highlighting the area, copying and pasting, searching, or opening the PDF in an editor that moves the shape reveals it. Changing a text colour to black on black has the same problem.
What a real redaction does
A real redaction tool deletes the underlying text, image or vector content in the marked area when you apply it, and then draws the box. After that the words do not exist in the file. Acrobat Pro has a Redact tool for this, and other PDF editors have similar commands. Look for an explicit Apply step: marking an area is not the same as applying the redaction.
How to check your result
- Open the redacted PDF and press Ctrl+A (or Command+A) to select all text. Redacted areas should not select.
- Search for a word you redacted. It should find nothing.
- Copy the page into a plain text editor and read what comes through.
- Save a copy and run the file through the File Metadata Viewer to look at what else the file records.
Redaction does not touch metadata
Redaction deals with the page content. The author, the program, the dates, the XMP and any earlier saved versions are separate. After redacting, clean them with the PDF Metadata Remover, which rewrites the file and keeps only what the current pages need.