Importing articles directly from Safari into ReadCube is very easy to do.  First, find the online PDF of the article you’d like in your mobile browser (most websites have mobile versions that make navigating to articles easier on a touchscreen).  Once you’ve found the PDF you’d like to download into ReadCube, choose the “export” icon in the top right and under the “open with” options, swipe to the right until you find ReadCube.  Choose this option and the PDF will be imported into ReadCube and available for your personal use.