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I don’t know if muPDF Viewer is, technically, free software, but i use it for the completely minimal UI. You can only read and search, it’s very limited in a good way.
It is, and you can get it on F-Droid. I like how quickly it loads while still having all necessary features.
You only need Librera Reader if you want an ebook library manager with dozens of customizable features. In which case, get it on F-Droid to get all of them for free.