It seemed that these were already removed when the PDF was rasterized, but now they're explicitly being removed.
For your friend's air gapped environment, the file works offline after the libraries cache on first load, but it does pull PDF.js and pdf-lib from CDN so a one-time internet connection is needed.
To run it fully offline you'd need to download those two libraries separately, transfer them to the air gapped machine, and swap the CDN links in the HTML to point to the local files instead.
The point about metadata is a good one, I checked a test file that I used and you can't see metadata from the original PDF, you only see basic info about the new PDF file and that it was produced by pdf-lib.
There definitely could be other things that a redactor should do, but for most use cases I think steganographically stored info lives outside of the threat model.
edit: ran strings on the output file, nothing but PDF structure and compressed image data, no original text content - thanks for the suggestion.
I recently needed to verify past employment and to do so I was going to upload paystubs from a previous employer, however I didn't want to share my salary in that role. I did a quick search online and most sites required sign-up or weren't clear about document privacy. I conceded and signed up for a free trial of Adobe Acrobat so I could use their PDF redaction feature. I figured there should...