I think that's actually hard... because you have to know what the text is you're redacting to get the spacing for the letters right. You want to redact specific words, but keep the rest of the document the same. This means that you need to know the letters (and their order due to kerning).
The best way to redact a PDF is with a printer, a sharpie marker, and a scanner.
The best way to redact a PDF is with a printer, a sharpie marker, and a scanner.