Every download link we issue already contains a unique code that expires after the number of download Attempts within the number of Hours you specify in the product's settings, so even if a link gets shared or out "in the wild" somehow, it soon becomes useless to everyone.
Also, the actual download links provided in our thank-you pages cannot be followed by search engine crawlers, and we've never seen a case where even the thank-you/download page itself has been indexed, nor should that happen.
Browser toolbars should not report every page visited for search indexing purposes, as search engines cannot reasonably presume every page a user visits is meant to be accessible to the general public; they generally rely on their own, automated "spider" programs that simply follow links from pages which are already public.