Why can't I down load a EPUB file?

What app you are trying to download it to ? Safari on the iPad doesn't support the downloading of content (the iPad doesn't have a 'file system' like a 'normal' computer, everything has to be stored within an app) - if you have the iBooks app on your iPad and it's a digital rights free epub, then you could try pressing and holding the epub and see if you get an 'open in' option.

Thanks King_Penguin! The proper app must be installed before iOS will allow you to download. Once I had iBooks installed it worked fine (other readers should work as well).