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.
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
redact on pc -> print -> scan
Pretty much as infallible and saves a huge amount of scissor time.