Note that device features are not detected, instead browser's viewport size is used to decide the UI sizing. So if you manually resize the browser window (or emulate a device via browser's F12 dev tools), phone or tablet size can be triggered.
The default is OnAny which means phones and tablets.
public DocumentViewerMobileMode MobileMode { get; set; }
Public Property MobileMode As DocumentViewerMobileMode
Get
Set