A redaction box hides content visually. On its own it does not remove the text underneath, which stays extractable by anyone who copies from the file.
To actually remove it, this editor rebuilds each redacted page as an image when you save. That page's text stops being selectable and any form fields on it become static. Pages you did not redact are untouched.
Draw in the box, then place it on the page. It is stored as lines, not a picture, so it stays sharp at any zoom.