There seem to be two audiences that you're trying to reach:

* Those that value the content.
* Those that might want to advertise.

Since you talk about "giving back" it isn't clear if you're really trying to make money doing this, or if the free download is a tactic to generate more advertising revenues.

To reach the users of your content, some good steps would be:

* Contact previous visitors (i.e. Those that paid to download, those that inquired but didn't purchase, or abandoned the download).

* Tie into educational forums and websites that provide homework advice, etc.

* Reach out to associations (school boards, teacher sites, etc.).

* Find influencers (bloggers and writers) that might find the content of interest.

Some press releases (web oriented) using a service like PRweb could really help raise visibility and attract the interest of influencers that follow your space.