Customising WordPress Search Results by Relevance

Today I setup 4 different search facilities on a single WordPress site. 1 full site search, and 3 other search facilities that search the contents of custom post types.

The search results are then ordered by relevance to the search term/keywords, rather than publish date as normal. I also included PDF uploads to be included in the search results, although at a lower priority to the website articles, events, and pages.