Good point, just pushed a fix. Title, author, subject, keywords, producer, creator, creation date, and modification date are now explicitly stripped from the output file metadata.
It seemed that these were already removed when the PDF was rasterized, but now they're explicitly being removed.
Just open sourced it: github.com/mr-guac/redactpdf
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.
Appreciate the feedback. The steps you listed are essentially what the site is doing. Upload a PDF, add the black boxes, it gets converted to PNG and back to a new PDF. The value of this tool is just to streamline that process to make it quicker and easier.
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.
One game that I thought had a great "pause" feature was Dragon's Dogma, you can pause the game mid fight to use items that heal or remove status effects. At first I felt this was kind of cheesy, but due to the games inventory weight limits it actually made traveling long distance or tackling dungeons more strategic because you had to bring the appropriate items for the enemies you'd be facing and use them sparingly or you'd be in trouble. Further into the game it actually felt like it required more strategy and planning, I thought it was pretty great game design and really liked it.