Work on a copy and use a true redaction operation, not a colored rectangle layered over text. After exporting, reopen the file in a different viewer. Try selecting, copying, searching, and extracting the area that was redacted.
Check every page, including thumbnails, comments, attachments, metadata, and previous revisions if the format or tool preserves them. For scanned pages, inspect the underlying image as well as text layers. If the information is highly sensitive, have another person verify the exported result before disclosure.
The appearance of a black box proves only that something is covered visually.
Akousa