Dropbox and Kobo Arc

HI
I have dropbox on both my computer and kobo arc. I have sent a link to myself for all my books on dropbox on my computer and they all show up on my dropbox on my kobo arc. When I try to download to my kobo arc I get "Can not open James Patterson - Guilty Wives.epub Please download an app that can open this file" Why can't I download these epub files to my kobo arc?
Thanks for your help