How to redact a PDF on Mac(Preview, and a safer option)
Every Mac comes with Preview, and it can black out a PDF. Whether it truly redacts the file or just covers the text depends on your macOS version and, more importantly, which tool you use.
Using Preview
Open the PDF, then look for the Redact option in the Tools menu or the markup toolbar. Recent versions of macOS have a real one that blacks out and removes the selected text. Select what you want gone, apply, and save (Preview may prompt you to save a redacted copy).
If the only thing you can find is a filled rectangle or shape, stop. That draws a box over text that stays selectable underneath, which isn't redaction.
The safer option
If you'd rather not gamble on which Preview tool you have, redact in your browser. Open the free tool in Safari or Chrome, box every sensitive bit, and apply. The text and images inside each box are deleted and the page is flattened. Your file stays on your Mac and is never uploaded.
Check it
Open the finished PDF, select all, copy, and paste into TextEdit in plain-text mode. If anything you redacted appears, it was only covered. The full reason.