var pdfViewer = hostBorder.Child as RadPdfViewer;

if (pdfViewer != null)
{
    var canvas = viewer.ChildrenOfType<Canvas>().FirstOrDefault(x => x.GetType().Name.Contains("ContentElementsCanvas"));                
    var visualTargetsDictionaryField = canvas.GetType().GetField("pageNumberToVisualTarget", BindingFlags.NonPublic | BindingFlags.Instance);
    var visualTargetsDictionary = (Dictionary<int, List<VisualTarget>>)visualTargetsDictionaryField.GetValue(canvas);
    foreach (KeyValuePair<int, List<VisualTarget>> target in visualTargetsDictionary)
    {
        for (int i = 0; i < target.Value.Count; i++)
        {
            VisualTarget item = target.Value[i];
            item.RootVisual = null;
            item.Dispose();
        }
    }  
}
hostBorder.Child = null;
hostBorder.Child = new RadPdfViewer() { Document = newDocument };

private void ReloadDocument()
{
	var navigationPane = this.pdfViewerNavigationPane;
	var navigationPaneTreeView = navigationPane.FindChildByType<RadTreeView>();
	if (navigationPaneTreeView != null)
	{
		object itemStorage = typeof(RadTreeView)
			.GetProperty("ItemStorage", BindingFlags.Instance | BindingFlags.NonPublic)
			.GetValue(navigationPaneTreeView);
		MethodInfo itemStorageClearMethod = itemStorage.GetType().GetMethod("Clear", BindingFlags.Instance | BindingFlags.NonPublic);
		itemStorageClearMethod.Invoke(itemStorage, null);		
	}
	
	this.DocumentSource = theNewDocumentSource;
}