Reader exports to an online Adobe application that requires a sign-up. The first two in the link I posted don't require an account or an email. Not a huge difference, but important to some. Personally, I use Adobe Pro - you can get it pretty cheap with a student/teacher discount.
Update: http

/help.adobe.com/en_US/reader/X/using/WS82af097c4236e2eb380f3d3d130e384098f-8000.html