To reproduce:
1. Add a RadPageView with no pages and set its Dock property to Fill.
2. Set the ViewMode property to ExplorerBar at design time.
3. Use the following code snippet:

private void AddOutputPage(int index)
{
    string title = "Test-Page " + index;
    RadPageViewPage page = new RadPageViewPage(title);
    this.radPageView1.Pages.Add(page); 
}

private void Form1_Load(object sender, EventArgs e)
{
    for (int i = 0; i < 20; ++i)
        AddOutputPage(i);
}

As a result the current page header at the top of the RadPageView is not displayed until you resize the pageview.