Also, make sure you black it out on the same layer. Plenty of people add a black box over something in a PDF and you can get the data just by selecting it, copying it, and pasting it.

Also also: black it out with an overly large box, otherwise it's possible to infer information from the length or height of the box. Eg, is the number two digits or ten? Box length might tell you.

The instructions we've used for years

1) Print it out 2) Cut out the parts you want to redact 3) Scan the paper

This might be more work, but 1) It's easy enough for anyone to understand 2) It's really, really hard to get wrong

A bit easier but more or less as effective is

redact on pc -> print -> scan

Pretty much as infallible and saves a huge amount of scissor time.

I saw someone recommend screenshotting the PDF to images, then converting those back to a PDF to ensure that you've scrubbed all non-visible artifacts.

macOS’s Preview (Big Sur and up) has a Tools / Redact menu item that automates this process.

And in this particular case the underlying text is visible even with naked eye.