Find their Transaction ID in either your PayPal account history or your E-junkie Transaction Log, then go to your E-junkie Seller Admin > "Re-activate expired links / re-send thank-you email" and paste in that Transaction ID where indicated, make sure "Re-send email to buyer" is checked, and click Submit.
Direct downloads to mobile devices can be a bit of a mixed bag depending on the particular device and/or what apps the buyer has installed on it, but they should at least be able to download the file to their regular computer and then sync the file to their mobile device from there. This related discussion covers the matter in more detail:
http://www.e-junkie.com//bb/topic/6281