Site search results

So I have about 20 categories on the site, one of them is Press Releases. I’ve been asked to make sure that posts in the press release category only show up in that single category BUT can also be found by using site search for other key terms like “compliance” for instance… even though compliance is not mentioned anywhere in the post. Which I am finding difficult. I thought I could simply add a compliance tag, but that does not show up in site search either. Do you have any Suggestions on how to get the post to show up when searching for a term that it’s copy does not contain and its not in a category of?