SlichoArt

Select the cube layer, go to fx, turn on outer shadow, click on it (ios app version) play around with settings below, you can add shadow, lighten/darker it, choose shadow colour and blending mode and add a blur. If you don't want it to be fixed on to the original cube (so a separate shadow layer) copy cube, make selection of cube, add new pixel layer and paint the selection in your shadow colour, you can later distort, deform, use perspective and blur fx to make it a fit.

You could also use the app iZip (free as well) I upload directly to my iCloud so I can use them anywhere.