I put those, and many more, on a Micro SD card (actually, several copies). It’s always in every e-reader in the house, and we have a solar charger for the e-readers. And ONE e-reader, just in case, is in the Faraday Box. . .
I am sort of low-tech. My Daughter could probably show me how to put them onto a disk of some kind. My eyes hurt if I read the computer or kindle for very long, so I often just pring stuff and put it in 3 ring binders.
However, it would be great to have a couple of back up disks that I could load on our portable computer.