Question

What is the difference between 

 PrepareImageMode.EnhanceLocalContrast

and

FRPage.ImageDocument.EnhanceLocalContrast()

Answer 

Basically, it's the same method that works in the same way but the way it is applied is different e.g. FRPage methods can be applied to a page separately from others during the processing whereas PrepareImageMode is applied to the whole document before processing. 

More info on that can be found in the documentation: 

FRPage

PrepareImageMode