Can't find .epub piles from Books in my iCloud Drive

So basically a few years ago, I downloaded a bunch of books as .epub files onto my phone. These were then synced with iCloud. Recently, I've been trying to find and send these .epub files to one of my friends, but they're nowhere to be found (on my PC or in iCloud). It's very strange, because I can download them to both my phone and iPad. Though more recent books I've download as .epubs I can easily find in my Files folder on iCloud, the older ones aren't there. It doesn't make any sense to me, where are these ghost files located?

For information: I've tried downloading iCloud Drive as an app on my PC, I've also downloaded the apps Apple Devices and iTunes, but neither of them show the files I need. I'm on IOS 17.7.1 and I must've downloaded those older .epub files from 2017-2020. Thanks in advance!