Securing static content

The best option is to integrate Marketo registration with an existing user store and use an authentication/authorization scheme that may exist as part of an intranet/extranet.  Put your download behind the authentication/authortization. Seemlessly Integrating marketo registration with an existing authorization/auhtentication scheme this is generally a tall request.

What we've done in the past is added some javascript to cookie someone when they register.  On the page with the download link we check for the existence of the cookie before displaying the download link.

Implementing the cookie check is only security by obfuscation and has many holes but it suited our needs.

Thank you Andy for your reply. Option number one is a TALL REQUEST :). Option two seems fine, but here is a thing. We already have some leads (for instance say 1000) in marketo db. We cannot ask them to re-register. Also marketo cookie will be already there (when lead first time registered). 

No reason to re-register.  You can check if the person is registered via the SOAP API.  A developer should be able to implement this in short order.

http://community.marketo.com/MarketoArticle?id=kA050000000KyqwCAC

One thing to keep in mind, no download is 100% secure.  If you offer a download like a PDF, once it is downloaded it is no longer secured.  It's a file that can be emailed as an attachment or redistributed on another website.  If you have content you want 100% secure, a download is probably not your best option.