When I was involved with a contract negotiation, they emailed me a pdf offer with addresses, names and important numbers retracted with black and asked if these terms are OK with me and we can talk about the numbers.

I noticed that when I select a region of text with a mouse, the retracted' black text becomes white text over black background. It revealed name of our competitor, addressees and numbers in their offer, the whole retraction was just a typographic trick.

When in doubt, print -> scan -> convert to pdf.