How should I announce that I'm changing my website from pay-per-download to a completely free model?
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* 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.