Unfortunately, Amazon is the only ereader manufacturer which supports storing Personal Documents on their cloud, and syncing them,
at all.
And they only support that for the hardware devices and mobile apps.
Cloud Reader, and Kindle for PC, are both out, as they only support downloading and syncing purchased content.
I suppose the theory is that your Personal Documents don't need to be synced to your computer, as you already have them.
Note: You can
open the ebook file (original copy, or downloaded though E-Ink device) in Kindle for PC. But that won't help you sync annotations.