This ability is available for free under GPL license (the exported files will have DHTMLX watermark).
If you purchase a license, then you can use the export service within the support period (1 month in
Commercial and 12 months in Enterprise license) without a watermark.
If you want to extend the period of using the export service without a watermark,
you can purchase a 12 months subscription for $70 per year for one domain.