assets.rb

ActionController::Base.asset_host = Proc.new { |source, request|
  if request.env["REQUEST_PATH"].include? ".pdf"
    "file://#{Rails.root.join('public')}"
  else
    "#{request.protocol}#{request.host_with_port}"
  end
}

Think again. Asset hosts are there in production to roll out things like cloudfront and multiple assets servers. It can be used to avoid pipelining of assets in the rowser also (using asset1, asset2 hosts for example) . This particular gist is used in production for 2 of our apps for more than a year without any glitches.

Thank you for the gist!
Here is correct version with slash at the end for root_url option. Works for me too on heroku.

PDFKit.configure do |config|
  config.root_url = "file://#{Rails.root.join('public')}/"
end

If you use the OP's method:

consider using this instead

if request.path.include? ".pdf"