How to save specific pages when saving a pdf?

I need to have the pdf automatically save only a couple of pages within a pdf. I know there is document will save inside document actions, but I am not sure what code I will need for this. When my clients use this pdf it needs to auto save or print  the specific pages I set.

I know there is , but it saves as a .tmp , is there a way to save it over my current pdf. Thanks alot.

this.extractPages({nStart:5});