Importing articles directly from Safari into ReadCube Papers 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 either the top right or bottom and under the “copy to” options, swipe to the right until you find ReadCube Papers.  Choose this option and the PDF will be imported into your ReadCube Papers library and available for your personal use.