gistfile1.rb

    pdf = Prawn::Document.new(:template => @format.zip.path)
    inspector = PDF::Inspector::Page.analyze(pdf.render)
    pdf.font 'Helvetica'
    pdf.font_size = 8
    1.upto(inspector.pages.size) do |page|
      pdf.go_to_page(page)
      pdf.transparent(0.2) do
        pdf.fill_color "ff0000"
        pdf.draw_text "watermark", :at => [200,420], :size => 32, :rotate => 45
      end
    end