Most surefire way would be to turn the pages into images, paint over the images and then stitch the images together as a new PDF. The low tech version would be to print them out, put something that's fully opaque over the undesirable sections and scan them back in.

Redacting by removing the text in place and putting black blocks isn't always enough either. Variable size fonts (as opposed to monospaced font) will leave gaps that can be potentially guessed by probabilities so that is something to take under consideration. You'll want to randomize the text to remove before you remove it and black it out.

Get a software that lets you edit, not just annotate, the text and remove it.