save pdf to file using template

I can create and save a pdf file with:

pdf = PDF::Writer.new() pdf.select_font "Times-Roman" pdf.text "Sample PDF Document", :font_size => 32, :justification => :center pdf.save_as("pdffilename.pdf")

but I'd like to re-use rpdf template, i I use the following code:

pdf_content = render_to_string(:template => "documents/#{template}-#{kind}", :layout => false) set_no_cache_headers send_data(pdf_content, :filename => "#{@document.name_for_file}.pdf", :type => 'application/pdf', :status => 200)

I have the error "Can only render or redirect once per action". How can I save pdf to server disk without render twice ?

thanks kind regards, Enrico.