PDF Export required a new method which will open on new tab instead of downloading it. eg: kendo.drawing.pdf.open(group,"Invoice.pdf")
You are right, this happens only when using FireFox, and might be browser-specific behavior when saving files. I reverted the status of the request.
The download dialog should prompt you whether you would like to open the file directly in a browser, or save it locally. You can test that on the following demo: http://demos.telerik.com/kendo-ui/pdf-export/index I think the first option is what you are after, let me know if not.
It is not prompting to save or open. I am using Google chrome browser. It directly download the file.