Have you ever searched for the phrase "click here" by mistake? If so, you probably wondered, how does Adobe Reader link get on top of the search results without any type of advertising from the vendor and the fact that your search phrase is not listed on that page at all. The answer lies deep beneath the surface of SEO (search engine optimization).
You must have seen pages with links to .PDF files, where webmasters offers a redirection to a free Adobe Reader tool for viewing such content. They write "If you need software to view this document click here" and turn the last two words into a hyperlink to Adobe Reader download site. Every time another webmaster uses the same strategy, search engine crawlers identify "click here" with a link to Adobe's site increasing corresponding PageRank.
Consequently, even though "click here" is not listed anywhere on the page, not in alt attributes of links or in meta tags, SEO algorithms rank the innocent Adobe Reader site the highest when searching for "click here".