Redirecting from a portlet to a servlet is not going to work. That is not what Shazzam meant when he (she?) answered your request at http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4155726.
Instead, you should set up the link that your portlet displays (the one that the user clicks to get the PDF) to go directly to the servlet, bypassing the portal entirely. The servlet can then reply with the PDF.