It is awful, the worst "search engine" which exists. I absolutely hate it and this is the only thing which wants to make me move away from Confluence. When you need it the most, and this happens often, you know that you definitely cannot rely on it. Any data you put in there is lost, unless you have a good hierarchy and know what to find where without relying on the search.
The search engine will happily search any attached pdfs and return those. It just won't search the actual Confluence pages, which seems like it would be easier.
