Yes, my technical stuff is organised by directories. But much of it has no meta data and is a mix of scans as DJVU, PDFs, TIF, jpeg and png. Only a tiny percentage that are either PDF books small enough for 8″ or PDF instruction manuals that are small are on Calibre.
The ebooks books are all by author directory by default, but that's irrelevant. My tablet does have ebooks and PDFs as two folders on the 256G uSD card and the PDFs are mostly just copied from the PC structure. I use Xodo for those. Various epub apps on Android phones & tablets just to test.
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