prawn/security :permissions

everyday Ruby 
#!/usr/bin/env ruby

require "prawn"
require "prawn/security"

pdf = Prawn::Document.new

pdf.text "Hello, world!"
pdf.encrypt_document :permissions => {
    :print_document     => false,
#   :modify_document    => false,
    :modify_contents    => false,
    :copy_contents      => false,
    :modify_annotations => false,
  }

pdf.render_file(File.basename($0, ".rb") + ".pdf")

「:modify_document」でなく「:modify_contents」が正しそう