ferrum.rb

format.pdf do
  html = ApplicationController.new.render_to_string(
    template: 'pdfs/test',
    formats: [:pdf],
    layout: 'layouts/pdf',
    assigns: { user: @user },
    encoding: 'UTF-8'
  )
  Ferrum::Browser.new(timeout: 7).tap do |browser|
    browser.content = html
    begin
      browser.evaluate_async(%(addEventListener('load', arguments[0]);), browser.timeout)
    rescue StandardError
      nil
    end

    browser.network.wait_for_idle
    pdf = browser.pdf(
      format: :letter,
      encoding: :binary,
      margin_left: 0,
      margin_right: 0,
      margin_top: 0,
      margin_bottom: 0,
      print_background: true
    )
    browser&.reset
    browser&.quit
    send_data(pdf, filename: 'test.pdf', type: 'application/pdf', disposition: 'inline')
  end
end

love it!