are all the PDFs gated on a marketo landing page? If so, the canned landing page performance report might be one way to get an idea (conversions).
if not, you could maybe track unique page views via google analytics?
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GA would be best.
You could also run CONTAINS .pdf on a smart list, but only the LP Perf report will show you a clear count by PDF. But I'm not sure this will work as you want it to.