According to the PDF specification (AdobeĀ® Portable Document Format Version 1.7), the appearance dictionary is optional and it is specifying how the annotation is presented visually on the page.
Exceptions thrown:
InvalidCastException: 'Unable to cast object of type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfNull' to type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Annotations.Appearance'.'
NullReferenceException: 'Object reference not set to an instance of an object.'