See:
http://devnet.kentico.com/Forums.aspx?forumid=48&threadid=12889 For a thread on mobile development.
It seems that officially the Kentico best practice suggests to duplicate your content for a mobile device - this is because each document may only have a single template.
But - you can do various other clever things to avoid duplicating your content:
1 - You could create a single page that takes a document Id as a parameter on the querystring and renders the contents of that document using a different template.
2 - You could use a few repeaters on the document template, one that shows the document for a desktop, but hides on a mobile, and another repeater that shows the mobile version and hides for the desktop. So you'd need some way to detect that a mobile browser was being used - cookies, querystring or path are the most obvious.
Lance