This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
This isn't a JSP issue--it's a browser issue. You need to have your browser configured correctly to automatically open the PDF file in the same window as the webpage instead of giving you the dialog box. As far as your code is concerned, it's correct.