But such a document is obviously unredacted. A black boxed PDF appears to be redacted, but isn't. Accidents happen.

Now that you've shifted the goalposts back closer to the original discussion, what's your point? Yes, you can leak the "nonexisting" file in multiple ways, including the printed one, and yes, "accidents" happen. So are they more likely to happen if you ban digital search and force paper and ink redaction instead? Are they more likely to happen if you black out digitally before printing or underline digitally and ink out physically?

And the "obvious word needle in a haystack of many thousands of pages" isn't as self-healing as you appear to think it is.