[BetterRed]

Calibre will process zips and rars as ebooks if they conform to certain structures. For purposes such as yours I avoid zip & rar and use another archive format such as 7z.

However if I have two pdf's (e.g. English and French) of the same book, as you appear to have, then before adding the second book I will rename it to '<whatever>.PDF2'. I then have to do the add via Add files to selected book. And in the OS I associate .PDF2 with my normal PDF reader

There are some things I can't do with the PDF2 from within the Calibre GUI, such as conversion, for which I would use the CLI; but many other operations such as send to device, save to disk etc should work. I also put a note in Comments to remind me what the PDF2 is - eg it's in French.

If you use Spotlight you might have to tell it to handle PDF2 files like PDF files.

I use 7z containers for things like multi file audio-books

BR