This is a known problem with iOS/Email on Acid. Possible fixes, as reported by Email on Acid support:
  1. Make sure that you have a minimum of 1,019 characters before your closing head tag (</head>) including spaces and carriage returns.
    If you don't have any need for more styles nor a style block, try inserting several lines of empty spaces.
  2. Another option is to remove your <head></head> tags entirely. Be careful with this solution, some email service providers might place head tags within your email dynamically.
  3. If your email is below 10k, you can try removing all carriage returns and double spaces. We've seen instances where the download button no longer showed up if the email was at or below 7k.
If you are experiencing this problem, please contact support.