If I were to tell paypal to direct the user to a page after the user a paid for a ebook with a url like:

http://www.somerandomsiteimade.com/c...ecure=jjf7474g

And that page contained code to check the url for information, and if that information was present it would allow them to download the ebook, however if the information was not present or incorrect the page would redirect the user..... would this be secure?

What are the possiblities of someone guessing that information and the location of the file?