Use the attached project to reproduce. 

Workaround:

var combo = toolbar.ChildrenOfType<PercentComboBox>().FirstOrDefault();

var binding = new System.Windows.Data.Binding();
binding.Path = new System.Windows.PropertyPath("ScaleFactor");
binding.Mode = System.Windows.Data.BindingMode.TwoWay;
binding.Source = pdfViewer;
binding.UpdateSourceTrigger = System.Windows.Data.UpdateSourceTrigger.PropertyChanged;
BindingOperations.SetBinding(combo, PercentComboBox.ValueProperty, binding);

var binding1 = new System.Windows.Data.Binding();
binding1.Source = pdfViewer;
BindingOperations.SetBinding(combo, PercentComboBox.RadPdfViewerProperty, binding);