To reproduce: 

for (int i = 0; i < 20; i++)
{
    this.radPageView1.Pages.Add(new RadPageViewPage("Page" + i));
}

this.radPageView1.ViewMode = PageViewMode.Backstage;

Only a few of the pages are visible and the user is not allowed to scroll to see the rest of them.

Workaround: 
1. Make use of the strip buttons:

            RadPageViewBackstageElement el = this.radPageView1.ViewElement as RadPageViewBackstageElement;
            el.ItemContainer.ButtonsPanel.Visibility = ElementVisibility.Visible; 

2. Place the control in RadScrollablePanel

 RadScrollablePanel scrollablePanel = new RadScrollablePanel();
            scrollablePanel.Dock =  DockStyle.Fill;
            this.Controls.Add(scrollablePanel);

            RadPageView pageView = new RadPageView();
            pageView.ViewMode = PageViewMode.Backstage;
            scrollablePanel.Controls.Add(pageView);

            for (int i = 0; i < 50; i++)
            {
                pageView.Pages.Add(new RadPageViewPage("Page " + i.ToString()));
            }

            
            pageView.Size = new System.Drawing.Size(1000,2500);