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How to redact a scanned PDF on a Mac

In a scan, the words are part of a picture, and a hidden text layer may hold them too. How to redact both on a Mac and prove it worked before you send it.

The problem

You need to hide a name or number on a scanned document before sending it, and you are not sure a black box on a picture really removes anything.

Can I redact a scanned PDF?

Yes, but there is a trap. The words you see are pixels in a picture, and many scanners also add an invisible text layer with the same words. Both have to go.

How do I redact a scanned PDF on a Mac?

  1. Work on a copy: select the file in Finder and press Command D.
  2. Open the copy in Preview, choose the Redact tool, and drag across each area to hide. On a scan you mark areas rather than selecting words.
  3. Save and close. Content marked for redaction is removed when the document is closed.

How do I check the redaction worked?

  • Look: the area is solid black.
  • Search: press Command F and search for a redacted word. If the scan had a text layer, it must not be found now.
  • Copy: select across the black area, copy, and paste into Notes. Nothing should come out.

Why can I still copy redacted text from a scan?

The text layer was not removed with the picture. Print the redacted file to PDF (File > Print > PDF > Save as PDF), which can drop a stale text layer, and check again. If you need it searchable afterwards, add text recognition after redacting, never before.

Our take

Run the search and the copy test every time, on the saved file. A redaction looks the same whether it worked or not; the two tests take ten seconds and are the only way to know what the person you send it to can find.

In Simply PDF

Simply PDF's Redact, with a key, removes the words it finds, blacks them out on the page, and reads the saved file back to confirm they are gone. Text documents rather than scans: redact a PDF.

Questions

Can I redact a scanned PDF?

Yes. Redact by area, then check that searching and copying no longer turn up the hidden words.

Why can I still copy redacted text from a scan?

The scan had an invisible text layer that was not removed. Print to PDF and check again, or redact with a tool that removes both.

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