The better news is that there are many of ways of emailing attachments without using GoodReader; many of them free. GoodReader is a favorite app for many users here, so it gets recommended often.
Not being able to attach directly in Mail is a consequence of Apple's sandboxing (security) approach to apps. The iPad requires a different approach to file management, where apps own their files. When you want to do anything with a file, you must start with the app that owns it. This is why almost every app that generates content has an email option built in, at a minimum.
There are two ways to get around this. One is to move most of your files to a cloud service and carefully choose apps that can access that service, or get another device that fits your expectations better.