Your second screenshot from a phone, the application is displayed inside the (default ?) web browser that may not display by default a page control. Is the web browser used to display your web app in the list of compatible browsers set by Xojo ? Not all browswers a are equal.
What we see in the screen shot, is it the PDF file or a Web Page from you app ? At the top right there seems to be a Hamburger menu, what happens if you tpuch it ?