How to redact a PDF safely
Remove the underlying content — a black box on top is not redaction.
Use a true redaction tool
Use Acrobat’s Redact tool (or an equivalent) that removes the content, not the drawing tools that just place a shape on top.
Apply redactions to delete content
After marking, run "Apply Redactions" — this step actually deletes the underlying text and image data, not just hides it.
Sanitize hidden data
Remove metadata, comments, hidden layers and attachments (Acrobat: "Sanitize Document") — sensitive data hides there too.
Re-open and test copy-paste
Open the output fresh and try to select/copy over the redacted areas. If any text comes out, the redaction failed.
Verify before release
Preflight detects an opaque box painted over still-extractable text and flags it as a botched redaction before the document ships.
FAQ
Why is a black box not a redaction?
A drawn box only covers the text visually. The text layer remains in the file, so copy-paste, search, and text extraction still return it.
How do I know my redaction worked?
Open the final file and try to select and copy the redacted area. If nothing is selectable, the content was removed. A checker can confirm no extractable text sits under a covering box.