[HELP]Viewer mode PAGEDOWN or PAGEUP?

[babyfacewaha]

In viewer mode setting auto image size as "fit image to window width" to better view the picture , if there's a vertical scroll bar then PAGEDOWN should means scroll to the below part of this picture in PDF reader while in XnView PAGEDOWN will go to the next picture ,
could you change this viewer way to familiar with other viewer? Thanks a lot!