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I've always operated under the assumption that nothing in a PDF is secure, and that the only safe way to redact something from one is to print it and rescan it. Convince me otherwise?

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I believe that if you edit the pdf so that the thing you are redacting is no longer in the pdf file, it is safely redacted. Printing a pdf and rescanning a pdf breaks text selection and screen readers, and basically makes it into a fancy PNG.

There is a bunch of data in PDFs that may have previously been visible but is no longer. I know that there are also fun meta-data attributes embedded on a per-page and per document basis - and also that PDFs can be portfolio documents where such a document will often contain no longer referenced PDFs that became irrelevant to the PDF. The file format is pretty crazy.

There are a few redaction tools that do exactly this, but in software.

You get a PNGish version, which you redact and combine back to PDF

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