One of ReadCube Pro's newest features is the ability to create references. While these files don't have PDFs attached to them, they can be used with SmartCite, are syncable, and can be formatted in over 35 different types. You can create these references by pressing the "Create Reference" button while in your library. It's the "+" icon in the top blue bar.
After you press this button, a window will pop up up with many text fields. First choose the type from the drop down menu, which will change the available fields, then fill in all of the appropriate text boxes with the available information.
At the bottom of the window - you have an opportunity to attach the document. Please note that it will need to be in .pdf format.
Once done, select the save button and the reference will be added to your library.