how to download file from document library programatically

Hi,

I have given whole path of my documents like on click of button

http://testserver:40001/Doclibr/test%20imtiaz.pdf.

Problem:

when user click on download button ,it opens file in new tab not download like this

I click any of this 

as I click any of this it open's file in new tab 

this is the code , I am using

   window.open(http://testserver:40001/Doclibr/test%20imtiaz.pdf, "_blank");

…

what to do to download it, instead of opening in new tab automatically???

Thanks