Preview seems like a good example of something that's worth open sourcing. Not only will people end up doing work for you, you get eyes on the code and more direct issue tracking.
Consumers get a product and they still have to go on Mac to use it.
Consumers get a product and they still have to go on Mac to use it.
My current solution is controlling a floating mpv window to open image, video or audio files as they are selected. This works well for A/V but not so well with other sorts of documents.
MuPDF is a great FOSS application and my go-to PDF reader. It lacks fancy annotation, and doesn't even have great text selection and copy/paste, but it is really fast, and has fast search, manipulation, etc.
https://developer.apple.com/documentation/pdfkit
Whether that could itself be open-sourced is an interesting question. (My concern would be that parts of it might be covered by Adobe NDAs.)
There will be ports to windows and linux in under a month.
I can't think of any company other than Apple that fit this description.