Sorry I forgot to get back to you. I've tried getting the direct PDF URL, but I can't seem to find it. The download page gives you a captcha and a submit button, but no matter what I download it with it does not show the actual PDF link, just the referrer URL.
You don't need to buy the book, it's free. Perhaps you clicked the wrong thing? Here is the direct link to the PDF download page of Treasure Island:
Link