The download feature is always in some state of broken, but it has gotten a lot better over the past couple of years. If you haven’t tried it in a year or so, you may have better luck now.

Sync thing is a great resource if you want your files to be replicated across systems. I use it for files that I might work on while in different locations and devices.

Its basically a personal onedrive or Dropbox.

I’ve used Ubooquity on and off for this. It has some nice features, but isn’t open source.

I don’t like Ubooquity, but Calibre somehow seems worse.