It is possible to use JavaScript to set the pixel y-offset of a UIWebView, e.g.:
[webView stringByEvaluatingJavaScriptFromString:[NSString strin
I think that you'll have to load and display the PDF yourself using the PDF APIs rather than UIWebView. Here is an possible idea of how to simulate snapping scrolling to rectangular grid boundaries which may help you make the experience more like a UIWebView for your users.
iPhone sdk Cocoa Touch - Pass touches down from parent UIView to child UIScrollview