This is basic Marketo Forms 2.0 functionality.
The simplest approach: set the form's Thank You Page ("Follow Up With..." in Form Editor) to the downloadable asset.
Recommended approach, since you will be implicitly verifying the lead's email: trigger on the Filled Out Form activity and send the asset via email. If you do this, don't send them a direct link to the asset but rather send them to a redirector page, as described here.
There are many resources on these topics. Start with the Marketo Docs and continue with a Community search.