The best way to improve Spotlight search is to control your metadata.
One reason that searches miss important files is that Spotlight generally indexes their content when it can. If you have one relevant document which still refers to
OS X rather than
macOS, but others refer to
macOS rather than
OS X, to hit both, you need to include both
OS X as search terms. We often don’t, and miss documents as a result.
Another problem with searches is that they return too many irrelevant hits, because you must use fairly generic search terms to ensure that you don’t miss relevant documents.
Search is hardest of all with non-textual material. Although some, such as images and music tracks, support their own metadata, we don’t make best use of them, and they don’t integrate well with indexed text content.
One solution to these and other problems – such as attaching copyright information to any form of document – is to store metadata in the extended attributes of documents. These are now supported on several file systems and operating systems. When chosen wisely, they also persist across iCloud Drive and similar.
Here are two free apps for working with metadata in extended attributes, so that they can enhance Spotlight search, and persist through iCloud: SearchKey and SearchKeyLite. Both are now offered in their first full release (having been in beta since the start of the year), and come complete with Tooltips, a detailed Help book, and PDF documentation.
SearchKey is the heavyweight of the two, which has a folder batch mode, can strip metadata, and more. SearchKeyLite is nimble, quick to use, and ideal for adding copyright information to a whole load of files, for example. If you use metadata seriously, then you’ll undoubtedly want both – they’re both completely free, and free of advertising or other nonsense.
I hope they help you.