ePub can do most of these things I believe; there are just not many “PDF-like” viewers for it, given that most existing ePubs are books.
Features I need are being able to quickly open multiple documents side-by-side; a feature I actively don’t want is maintaining some sort of “library” for me.
That's just the UX of most ebook reader application, it's not a limitation of the format itself. The conceptual model of the most popular epub readers are built upon the concept of a personal library, but that's just because, as you said: "most existing epubs are books". There's nothing stopping anyone from creating an epub file that's not a transcription of a book.
Calibre's e-book viewer should cover your use-case just fine, and in KDE is quite easy to set that viewer as the default application for epub files.
I don’t want pagination for reference documents, for one thing – I want fast seamless scrolling.
Not sure if that’s still the case, but on macOS, it used to be hard to open more than one ePub file at a time using Calibre too – also an essential feature for research.
Finally, Calibre viewer edits each opened ePub by inserting a “last reading location” metadata file by default! It’s possible to deactivate in the settings, but I need to remember to do it for every new installation. It’s an absolute no-go as a default for a document viewer.
I wouldn't assume that any application doesn't edit documents, especially metadata. My PDF application remembers the last location and zoom level viewed, though I don't know how.
Not just KDE, any Linux desktop should be able to do this. It certainly works perfectly well in Linux Mint Cinnamon and Xfce. It's a long time since I used Windows but I think it works there too.
> Features I need are being able to quickly open multiple documents side-by-side
I don't quite understand: Highlight the desired files in the file manager and press enter? Tile the windows that open? I am missing something (obviously) ...
Features I need are being able to quickly open multiple documents side-by-side; a feature I actively don’t want is maintaining some sort of “library” for me.