尝试使用所有rails控制器都具有的send_data方法。
就像这样：

# Generate a PDF document with information on the client and return it.
# The user will get the PDF as a file download.
def download_pdf
  send_data(generate_pdf, :filename => "my_file.pdf", :type => "application/pdf")
end

假设它是一个简单的机架终点：

class PdfEndpoint
  def call(env)
    [200, {"Content-Type" => "application/pdf", "Content-Disposition" => "attachment", "filename" => "filename.pdf"}, pdf_data]
  end
end

下面的Railscast可能会有所帮助：
http://railscasts.com/episodes/151-rack-middleware

如果控制器操作使用send_data，则选项:disposition应等于**“attachment”**（而不是“inline”）。
举个例子

def show
  client = Client.find(params[:id])
  respond_to do |format|
    format.pdf do
      send_data(generate_pdf(client), filename: "#{client.name}.pdf", type: 'application/pdf', disposition: 'attachement')
    end
  end
end

要生成PDF，可以使用Prawn。