void ManipulationDelta(object sender, ManipulationDeltaEventArgs e)
        {
            int idx = PDFviewer.CurrentIndex;
            var mousePoint = e.ManipulationOrigin;
            var leftTopPagePoint = PDFviewer.ClientToPage(idx, new Point(0, 0));
            var pagePoint = PDFviewer.ClientToPage(idx, mousePoint);

            if (e.DeltaManipulation.Scale.X < 1 && PDFviewer.Zoom > 0.4)
                PDFviewer.Zoom /= zoomFactor;
            else if (e.DeltaManipulation.Scale.X > 1)
                PDFviewer.Zoom *= zoomFactor;
            PDFviewer.SizeMode = SizeModes.Zoom;

            var newPagePoint = PDFviewer.ClientToPage(idx, mousePoint);
            var offsetPoint = new Point(newPagePoint.X - pagePoint.X, newPagePoint.Y - pagePoint.Y);

            var scrollPoint = new Point(leftTopPagePoint.X - offsetPoint.X, leftTopPagePoint.Y - offsetPoint.Y);

            PDFviewer.ScrollToPoint(idx, scrollPoint);
        }