[Jafo]

Hi all,
I've read through some threads but I still don't understand what I'm doing wrong. I have an ePub book that has left and right margins that are too wide for me. I've gone to Page Setup in Calibre and lowered the values all the way down to zero but it makes no change at all when I open the book on my Kobo. Am I doing something wrong or is there just some epub's that can't be modified?
Thanks