Page up/down with smooth scrolling?

Smooth scrolling is dependent on the application. TextEdit does offer smooth scrolling when paging through a long document. BBEdit 11 does not.

Apple's developer WWDC 2013 talk below is aimed at helping developers to achieve this:

Optimizing Drawing and Scrolling on OS X

Creating a responsive app requires fast drawing and smooth scrolling. Discover techniques to optimize drawing, find out best practices for handling layers, and learn how to combine those techniques with new features to achieve smooth scrolling.

If possible, provide feedback to the editor's developer that you value this behaviour.