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.

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.

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