Please note that you will be unable to change the page direction while using the infinite page size.
To change the page orientation between Portrait and Landscape, bring up the app bar menu by swiping from the bottom of the screen (Windows) or by tapping the three dots in the lower right corner (Windows Phone). Then tap Page settings, and select a page orientation by tapping on the current page orientation. Then tap Done (checkmark icon) to save the changes.
Please note that you will be unable to change the page orientation while using the infinite page size.