Vertical scrolling of pdfs in iBooks?

When I read pdfs via iBooks I tend to zoom a bit to get the page to a comfortable size. However, I cannot find a way to enable what is termed "Continuous Scroll" as is done in Mac OS X. Instead I am forced to swipe right to left to turn the page and then have to rezoom to the top of the new page. Am I missing an option somewhere? Continuous scroll would make viewing pdfs in ibooks 10x more enjoyable.