When you select a digital book, you will see in the "Reading Options" section that there are two viewers available. Digital titles will open in either viewer. 

Sign in and select the link of the viewer to open the digital book. Feel free to test out both viewers and see which you prefer. 

The Primary viewer allows more options for your reading layout and allows you to search within the text of a digital file. The primary viewer may work better on mobile devices or devices with a small screen size. 

The Secondary viewer generally provides a better full-screen experience with the ability to zoom in/out of text allowing you to find a comfortable view for your reading experience.