[syhprum (OP)]

I see two possibilities if the producer has the plaintext to hand he can replace each character with a black one then there is a small chance the plaintext can be recovered but for maximum security the document should
be retyped .

[Bored chemist]

The traditional answer is you use a black marker pen, then you photocopy it and send out the copy.

[evan_au]

Presumably it would be a built-in function in a secure text editor? Just highlight the text, and click "redact" (or ctrl-r)?

In the old days, they used a scalpel - letters arriving home from soldiers at the front would arrive looking like swiss cheese (but with rectangular holes)...

[SeanB]

Stupid version is to use Adobe's full suite and overlay the redacted sections with black, and then export it without first flattening it, which would make the characters disappear into the black. If not flattenned, you can simply remove the mask layer, and get the text back, which is often the step forgotten by most people doing this.