how to know if the pdf file is opened.

Hi,
on my web site I have many pages containing hundrend of url pointing to the pdf documents.
The problem is how I can know that the pdf file was open or downloaded by user ( I used session for this task). I have to point out that I did not integrate in my page any javascript action on those documents.
This point will help me to solve some historical access on the web site.
Thank you

This may be a kluge, but you could have the links to the pdfs point to a servlet instead of directly to the pdfs. When the link is clicked you could have the servlet record the fact that the pdf has been accessed and then forward to the pdf.