Looking for a PDF app with highlight and export function

Well I'm an university student and I have to read many journals

I have a kindle but it bothers me that it is very difficult to highlight the journals

I buy an iPad mainly to do this...

So what I'm asking is, is there an app out there that allows me to highlight a certain part of the document, and save the highlighted part into a separate file and then export it? I only want the extracted parts, not the whole document with highlights.

I know there are many PDF annotation/highlight apps out there, but I couldn't find one that does what I want. Maybe I'm missing something...?

All help would be appreciated. Thanks.