When importing a document with a missing state separator (e.g. linefeed) in the object stream (ObjStm) an exception is thrown: System.InvalidCastException: 'Unable to cast object of type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfDictionary' to type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfInt'.'

А possible workaround could be handling the exceptions: Handling Exceptions.