The export mechanism of RadGridView does not support exporting using templates. You can achieve this by using the printing functionality RadControls offer and redirecting the print job to a driver that will produce a pdf file rather than sending it to a real printer. I have attached an example project which demonstrates how you can customize the printing. There is also code showing how to use a pdf print driver to directly print to a file instead of showing print dialogs.
I hope this will be useful. Should you have further questions, I would be glad to help.