I'm not sure where this question belongs, but this looked like my best bet.
I have a site that offers free PDF downloads in a specialized area. There is a generic version and a custom version for each offering. About 30,000 downloads per month. I'm guessing that the majority of the PDFs are printed out and handed out to multiple people.
It occurred to me that I could put a 'sponsored by' notice at the bottom of each page, or just a plain one-line ad.
There would be no way for an advertiser to track the effectiveness of the ads, but on the other hand you are putting your ad literally in the hands of interested people.
Does this make any sense? If so, how could I sell this space? Who should I approach? How much should I ask?