No and/or Yes.
Yuzu for Mobile (iOS and Android):
No - For our iOS and Android app customers, once you have downloaded the eTextbook to your device you will not need to be online to read or annotate (notes & highlights) your digital content. We do recommend regularly accessing the Yuzu App when online to sync your Highlights and Notes. They are stored on your device until you sync.
Yuzu for WebReader:
Yes - For our WebReader customers, your digital library is stored online and your computer will need to have a live internet connection to access and read your eTextbook. Using the browser based application the digital content is not downloaded to your computer. Your annotations are automatically synced as you turn the pages.