In light of the news stories from the US about Paul Manafort's lawyers submitting a redacted PDF that could actually still be read, it might be no harm to dig a little deeper.
The PDF in question featured black boxes over the redacted text intended to obscure it, which can be done in any number of ways depending on the software you use. But the text underneath these boxes is still there and can still be copied/pasted to another application to be read. Some suggest that if you then 'flatten' the PDF that this will solve the problem, but that's not always the case. The same is true if you are working in MS Word and just change the background colour of the text to black.
Adobe's pro version of it's popular PDF reader Acrobat Pro contains a redaction tool (https://helpx.adobe.com/acrobat/using/removing-sensitive-content-pdfs.html) but I haven't tried it and would be wary of any software solution for something as sensitive as legal documents.
I would prefer a lo-tec but bullet proof solution like either taking a screenshot/s of the newly redacted document and saving those as a new PDF or printing the document out, going through it and redacting passages by hand with a black marker, scanning that and saving those as a new PDF. Both of these ways produce an image file rather than a text file so there is no actual text for someone to copy. One caveat if you are going the marker route - make sure its a good solid black as image enhancement software could be used to 'rescue' the text from beneath a nearly black or drying out marker stroke. Finally I would also save the resulting scan as a 'bitmap' rather than a 'greyscale' image, if your software will let you, as this will be just pure black and pure white with no greys at all.
Scan before and after image enhancement
