Use contentOffset
to achieve that:
Add scroll offest to x and y touches:
CGFloat xOffset = _myScrollView.contentOffset.x;
CGFloat yOffset = _myScrollView.contentOffset.y;
Then take of it the position of the scrollview:
CGRect frame = _myScrollView.frame;
All:
CGFloat pdfTouchX = screenTouchX - frame.origin.x + xOffset;
CGFloat pdfTouchY = screenTouchY - frame.origin.y + yOffset;