Google Site Search offers two features to customize search results to make sure items you want featured reach the top of the results list. “With "top result" biasing, you can target the top search results from specific sections of your website (product catalog or newsletter sign up page etc.) to make sure visitors can find the most relevant pages within your site. You can also organize search results based on the age of the documents with "date biasing". If you want to make sure that, say, a new PDF makes it to the top of the results rather than an outdated version, you can switch on date biasing and decide the level of influence (low, medium, high or maximum) so visitors can easily find the most recent version,” Google Enterprise blog.