PDFKit on Heroku with single Dyno

orders_controller.rb

# ...

def show_pdf
  @order = Order.find_by_token(params[:id])
  if @order.nil?
    flash[:error] = "Couldn't find invoice."
    redirect_to root_path
  else
    pdf = PDFKit.new(render_to_string(:action => 'show', :id => @order.token, :layout => "invoice"))
    pdf.stylesheets << "#{Rails.root}/public/stylesheets/invoice.css"
    send_data(pdf.to_pdf, :type => 'application/pdf', :disposition => 'inline')
  end
end

# ...