I don’t know about recent Kobo models, but I’ve had several over the years, and every one of them worked like a plain USB flashdrive when plugged in to a computer. Plug in cable, copy epubs just like you would to a flashdrive, unplug cable, then the device refreshes itself. No app needed. Any OS that understands USB mass storage (which is pretty much everything) works with all the Kobos I’ve had.

They can read PDFs, though it’s not as good an experience as a tablet. That’s more to do with the device’s smaller screen size, lower CPU power, and the limited-grayscale e-ink tech. It’s not a knock against the software’s PDF capabilities themselves which are actually pretty decent. Fortunately Kobos are pretty much intended as epub readers, and epub is a widely used and open format.

There’s one caveat with Kobos: last I checked, the first-time setup does require a Kobo account and wifi connection. After that you can just turn off the wifi permanently and copy files like described above though. My current one (the H2O Aura, which I think has been replaced by newer models) has never needed an active internet connection since that initial setup.