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 };

public class MyPdfViewer : RadPdfViewer
{
    public PdfDocumentSource PdfDocumentSource
    {
        get { return (PdfDocumentSource)GetValue(PdfDocumentSourceProperty); }
        set { SetValue(PdfDocumentSourceProperty, value); }
    }

    public static readonly DependencyProperty PdfDocumentSourceProperty =
        DependencyProperty.Register("PdfDocumentSource", typeof(PdfDocumentSource), typeof(MyPdfViewer), new PropertyMetadata(new PropertyChangedCallback(OnDocumentSourceChanged)));

    private static void OnDocumentSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        MyPdfViewer viewer = (MyPdfViewer)d;

        viewer.SetDocumentSource((PdfDocumentSource)e.NewValue);
    }
}

<local:MyPdfViewer x:Name="pdfViewer" PdfDocumentSource="{Binding MyViewModelPropertyOfTypePdfDocumentSource}" />