Blacking out a part of a PDF, or redaction of text What methods are there to blackout a part of a PDF?

I don't want to simply use Preview's annotation tools and then save the document, since then someone could open up the PDF and remove the annotation to reveal the part of the document which I'm trying to black out.
If you are looking for a no-cost way in Preview.app


File->Export (or save as)
Export PDF to TIFF file format (it's a multipage image format)
Open TIFF file
Export TIFF to PDF file format


Maintains page structure, OCR will be lost.