You're getting that error because it's a meta package-a package that represents a collection of packages-so you need to actually remove the set of packages it installed and replace it with another kernel. Simply installing an alternate kernel package should configure it to be the default booting kernel on Fedora. You want a replacement kernel to be installed before manually removing whichever is active right now.
Don't know about PDF viewers, but check the software store. I know there are a lot.