How to have apple iBooks or Kindle synced between devices?

There are books that I cannot obtain in the bookstore or amazon prime. For those books I'd like to be able to access them wherever I am (from all my devices, even if I get a new mac book). I also wish to be able to save all the notes in them and have that accessible. I assume one would new to sync in the cloud. Is this possible? I see the pdfs are in my computer's readers but I cannot have them sync between devices like, say, mendeley or evernote would. kindle isn't important. **What is important is that my pdfs are available everywhere.** Ideally backup with annotations. That doesn't seem to work on either. Thats the crux of my question. Which app is mostly irrelevant. Note that I strongly prefer to have have it work on 1) kindle or 2)iBooks in that order if possible but if it works on either that would be fantastic. ---- related: - https://apple.stackexchange.com/questions/49831/syncing-non-amazon-purchased-ebooks-between-kindle-ipad-and-iphone