Tell When a UIPageViewController is Scrolling (for Parallax Scrolling of an Image)

for (UIView *view in self.pageViewController.view.subviews) {
    if ([view isKindOfClass:[UIScrollView class]]) {
         [(UIScrollView *)view setDelegate:self];
    }
} 

this gives you access to all standard scroll view API methods.
And this is not using private Apple API’s.

I added traversing through subviews, to 100% find the UIPageViewController‘s inner scroll view
WARNING:
Be careful with scrollview.contentOffset. It resets as the controller scrolls to new pages

If you need persision scrollview offset tracking and stuff like that, it would be better to use a UICollectionViewController with cells sized as the collection view itself and paging enabled.