Birt report once generated the pdf in pdf file click on download it is showing only design file name

Raon

Birt report once generated the pdf file click on download it is showing only design file name I want to set input parameter as a Invoice no so that one I want to set as a download file name like in parameter is 123456 then my download filename is 123456.pdf how to do in birt report because I am using bilt in run servlet and birt version is 4.9 any one can help regarding this..