http://docs.webix.com/samples/15_datatable/30_treetable/07_pdf.html has export: true defined on the treetable, but export is not documented in the API, and the same code works with that line removed, or even with export: false.
Actually, http://docs.webix.com/samples/15_datatable/30_treetable/12_export.html is a better example because it showcases both PDF and Excel export, and also the exportAsTree option. The first example seems redundant and could be deleted.