There are 2 ways to present a file to a subscriber after opt in:
1. Put the URL of your download page URL at the 'Thank You Page URL' or 'Redirect URL' of your opt in form setting. You may also use a direct download link to your file.
For API integration:
For Email Data Integration:
2. Set this up at your autoresponder's side.
If you are using MailChimp, you can check it here: http://kb.mailchimp.com/campaigns/images-videos-files/send-a-file-to-new-subscribers
For constant contact, this link here might be a guide for implementing what you would want: http://howto.digioh.com/how-to-send-a-secure-file-after-someone-signs-up-on-your-constant-contact-form/
In case neither of the above is your autoresponder, please speak with your email marketing provider for more information on this topic implementation.