Your iPad needs to be set to Landscape Mode before the “Open as PDF” button will become active. You will not be able to view a PDF in Portrait Mode as there is too much content on the screen. Simply turn your iPad to its side, it will automatically switch to Landscape Mode and you’ll have access to all PDF features. If the iPad doesn’t automatically switch from Portrait to Landscape Mode, it’s because the Screen rotation lock switch has been activated. If that’s the case, using the Control Centre, swipe up from the bottom edge of the screen and search for the refresh button. Press it and you will see the words Orientation Lock: Off. Tap anywhere on the screen to close the dock.