unexpected download popup

I'm creating some pdfs using PDF::Writer:

def my_action
  invoice_pdf = render_to_string(:action => '../orders/
invoice.rpdf', :layout => false)
  path = ...
  File.open(path, "wb") {|f| f.write invoice_pdf}
end

At the end of the action, my_action.html.erb should be rendered. But
I'm getting an unexpected download popup:

(controller-name).pdf
type: Adobe Acrobat Document, 11.2 kb

thanks,
Luma