Is there a way to adjust search result to show documents first?
SharePoint 2010 enterprise search
Creating a custom ranking is possible - but its a science and lots of work.
Here are two posts that tell you about the background and how to customize it: http://calvisblog.wordpress.com/2010/06/21/custom-ranking-models-with-sharepoint-2010-background-value-and-administrative-overview/
I would not touch it if there is no very good reason.