found my problem

forgot the header stuff

 def pdf

  if @request.env['HTTP_USER_AGENT'] =~ /msie/i
     @headers['Pragma'] = ''
     @headers['Cache-Control'] = ''
   else
     @headers['Pragma'] = 'no-cache'
     @headers['Cache-Control'] = 'no-cache, must-revalidate'
   end


  send_data gen_pdf, :filename => "robbyonrails-fpdf-test.pdf", :type =>
"application/pdf"
 end