changing_margins.rb

require 'prawn'

class ChangingMarginDocument < Prawn::Document
  def start_new_page(options={})
    if (page_count % 2).zero?
      super(:left_margin => 72, :right_margin => 36)
    else
      super(:left_margin => 36, :right_margin => 72)
    end
  end
end

ChangingMarginDocument.generate "margin.pdf" do
  6.times do |i|
    stroke_bounds
    start_new_page unless i == 5
  end
end