Hey Dave if you have the number of visits and the number of downloads can't you just calculate out the value you want, using JQuery if you want to display on the site, if GA can't provide it directly?
Or am I misinterpreting what you want?
Doesn't that assume that all downloads originate from the page on which Dave has hosted a link though?
For example I can link to
Dave's free excerpt from here and someone clicking on it would register as a view/download of that file, but it wouldn't have originated from
the page Dave has linked it from though so any percentage calculated in this suggested manner would be skewed (and you could potentially have > 100% of downloads of the PDF compared to page views if more traffic is coming from outside the site).
I don't use Google Analytics at all, but is it not possible to see where each page/files referrers pages come from? Something like
this perhaps?