Simple noobie question... Dectect document open, call a URL

Hi,

I've been searching around and I can't seem to find out if this is possible.

What I want to do, is on a pdf open (on the destkop / or browser), detect that it has been opened, and then call a URL.

I want to call the URL so I can add a counter in my database to track the opens on my pdf.

Is this possible? If so will the user get any security warnings?

Any help would be very much appreciated.

Many thanks,

Brett