pageDown() --> self

        Repositions the data source downward and refills the display.  If the
        data source is already at the logical end of file (i.e., the
        bottommost available record is already shown), the browse cursor is
        simply moved to the lowermost row containing data.  If the data
        source is already at the logical end of file and the browse cursor is
        already on the bottom row, TBrowse:hitBottom is set true (.T.).