How do I email a PDF generated using PDFKit and on Heroku in Rails?

I am using PDFKit on Heroku (and also locally for dev) and am having difficulty emailing the generating PDF from my cron task.

This is the code in my cron job:

     kit = PDFKit.new(html_file) 
     file = kit.to_pdf 

     OutboundMailer.deliver_pdf_email(file) 
     #kit.render_file("pdf_from_cron.pdf") 

This is the code in my Mailer:

 def pdf_email(pdf)
  subject     "This is your batch of letters"
  recipients  "helloworld@gmail.com"
  from        "B开发者_开发知识库atch Email <hello@batch.com>"
  sent_on     Date.today
  body        "This is the body of pdf"

  attachment "application/pdf" do |a|
    a.filename = "batch-letters.pdf"
    a.body = pdf
  end

end

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I setup PDFKit on Heroku once and followed: http://blog.mattgornick.com/using-pdfkit-on-heroku. You may need to include the binary.