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I built a redaction process for a small company once.

My critical security step was rendering out the PDF as individual flat image files, then re-assembling it like a traditionally photo-copied document.

That way the loss-full operation is enforced, at the cost of forcing end users to OCR unsearchable image-scan (like) PDFs.

It's the only way to be sure!

You could replace the censored text with gibberish before blacking it out. Or some secret message for QAnons to find.

well if you had an open (.odf) or openable (.doc≈xml) format you could replace strings and be sure something was gone