This is something that you cannot absolutely control with HTML itself.
If the user is having a browser with PDF reading capabilities (or a plugin) and the corresponding settings to open PDF files in-browser, the PDF will open like that.
The PDF opens in a new tab simple because of your target="_blank", which has nothing to do with a download prompt.
If you are using HTML5 you can use the download attribute:
Download
If you have a back-end service which you can control or you feel like fiddling with your Web Server, you can always look for setting the right Content-Disposition. See this SO question for some nice discussion on Content-Disposition.
