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.
Why not handle all these two actions in Servlet on click of Button ? Ajax too could give you same problem (PDF document is opening before the validation) , might be synchronizing Ajax call can help you.