A Page contains multiple GUI objects (such as Buttons, Images, Sliders, etc), sized to match the Project Page Size.

Each page has two orientations, Portrait and Landscape. Portrait mode is defined as the orientation where the height is larger than the width, and Landscape is when the width is larger than the height.

Depending on the Viewer software being used, the page orientation will automatically rotate to match the orientation of the device. This is true for Apple iOS devices, such as the iPad and Android devices.
To stop a page from automatically rotating, all you need to do is ensure you don't have any GUI objects within the orientation you don't want to rotate to.