No, the docs say that those are values pre-defined by ActionSupport (technically the Action interface). That says nothing about *having* to return them. They're result names: you can name your results "fred" if you want.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com
subject: Return Types in a Action class from a excute method.