1. If they are all PDF you could use "clicked link" and "clicked email" with link containing ".pdf".
2. If you have Salesforce you could create a campaign for each document. Create a trigger for each thing you do and add someone to the sf campaign when they download. Then check whcih sf campaign has the most members.
3. We are now creating a marketo program for each content item (recorded webinar, whitepaper, etc). Program type is "Content" and with custom progression statusses (form fill or download equals succes). Visiting the form or getting the teaser email makes you a member of the program. This enables you to see for each lead who encountered any content item and if they succesfully downloaded it. This not only shows you which content items have the most interest and succes. But it also is a tool to determine and learn more about the lead (what is his interest, industry, challenge, preferred content format). The new marketo nurture tracks make it very easy to pull in the teaser emails from the programs (launch June 2013). I will create a presentation about this method after the summer for sharing with the community.
If you have any questions just email me at diederik.martens@quintiq.com
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