Can I still use apps on iphone 3g?

I just want to be able to read epub and pdf on my iphone 3g. It's jailbroken from 4 years ago, I don't know what the new think is now adays to make it use apps. All the apps in the app store doesn't work on iphone 3g anymore

Only apps that work with 4.2.1 or lower would work. Unfortunately most of the apps require 4.3 or greater.