Webview (or) Zoomable image inside scroll view - Vertical zooming is not working in Android

    <ScrollView VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand" x:Name="hello">
                      <Grid> 
                        <BoxView BackgroundColor="Red" Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="10" HeightRequest="200"/>
                        <BoxView BackgroundColor="Black" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="10" HeightRequest="200"/>
                        <BoxView BackgroundColor="Yellow" Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="10" HeightRequest="200"/>
                        <WebView HorizontalOptions="FillAndExpand" Grid.Row="3" Grid.Column="0" Grid.ColumnSpan="10" x:Name="pdfviewer" HeightRequest="500" WidthRequest="500" VerticalOptions="Center" />
                        </Grid>
    </ScrollView>