Hi Luis,
There are many ways to do this ... here's one.
Keep a text file on your site called hits.txt, that just stores a the number of hits so far.
The link could not go direct to the actual pdf but a php script that updates the hits.txt and then re-directs straight to the pdf using the header() function. The 'update' script could check the current value first, and re-direct somewhere else if the max is reached.
Paul.