There isn't a PDF that I know of. Most of the features in PS Touch have callouts the first time you use them.
If you've got a basic handle on Photoshop for the desktop, then you have a headstart on PS Touch. Still, you have to think a little differently. e.g., You cannot use non-destructive layers like adjustment layers yet (though I'm not sure that's needed; imported images are simply copied when pulled into a project anyway).