By default, the AsScreen property is False. Use the AsScreen property to have the page's preview closer with what displayed on the screen. This property is useful, when the control displays semi-transparent objects and displaying the preview may differ on what you have seen on the screen. 

For instance, the following picture shows the control with multiple semi-transparent bars:

 

If the AsScreen propetry is True, the preview of the bars looks as follow:

If the AsScreen propetry is False, the preview of the bars looks as follow:

Another difference that may be observed when using the AsScreen property is zooming the preview ie if the AsScreen property is True, the magnified preview may look stretched, as you can see in the following screen shots:

The following screen shot shows the preview of 200% when AsScreen property is False:

The following screen shot shows the preview of 200% when AsScreen property is True: