When you first open any PDF on an Android device from an outside source such as from your email, you'll be asked in what program you'd like to open that PDF. There is an option at the bottom of that screen - "always" - which, if chosen, will prevent the list of available PDF readers, including ReadCube, from opening again:



In order to un-do this, head to your settings, into apps, all, and then from the "3 dot" menu option on the top right, select reset app preferences.

Then, when you attempt to open the PDF again, the list of available readers will appear and you can select ReadCube.