Since the [!:PDFExportOptions.FileName] is not serialized, when [!:Combine] is . (which means all the views would be exported into one PDF file, whose file name is specified by [!:PDFExportOptions.FileName]), the returned PDFExportOptions would have [!:PDFExportOptions.FileName] as an empty string. The user has to set the [!:PDFExportOptions.FileName] before calling the [!:Document.Export(string, IList{ElementId}, PDFExportOption)] API exporting a “combined” PDF file.