C#

private void btnPageDown_Click(object sender, EventArgs e)
{
    Point p = webBrowser.AutoScrollOffset; // at first it returns {X=0,Y=0}

    y = y + 50;

    webBrowser.Document.Window.ScrollTo(x, y);  // it scrolls down the page

    Point p2 = webBrowser.AutoScrollOffset;     // and it returns {X=0,Y=0} again
}

x = webBrowser.DisplayRectangle.X;
y = webBrowser.DisplayRectangle.Y;

private void btnPageDown_Click(object sender, EventArgs e)
{

    int xx = webBrowser.DisplayRectangle.X;   // first it returns X=0
    int yy = webBrowser.DisplayRectangle.Y;   // first it returns Y=0

    Point p = webBrowser.AutoScrollOffset;
    y = y + 50;
    p.X = x;
    p.Y = y;
    webBrowser.Document.Window.ScrollTo(p);

    // y = y + 50;
    // webBrowser.Document.Window.ScrollTo(x, y); it also does work

    int xxxx = webBrowser.DisplayRectangle.X;  // again it returns X=0
    int yyyy = webBrowser.DisplayRectangle.Y;  // again it returns Y=0
}