Global.zoomLevel is the max zoom level i.e. 15 means 15x(2000%) zooming.
The page scale is calculated based on the pdf's max page size (width and height) and the view mode (vertical, single, double)
To change the scale of the page, you need to change the calculations done in the vLayout of the needed mode (PDFLayoutDual or PDFLayoutVert)
You may need to calculate the scale for every page based on its height/width, and save it into an array for example.