I second the recommendation by @NiranS of DEVONthink Pro. It is a very flexible database app for macOS and iOS that can sync data (though I only use the macOS app). DEVONthink can either “import” files (that is, manage files in an internal folder structure in its own database package) or “index” files (that is, work with files that are stored outside the database package but indexed by the database).
I choose to “index” files in DEVONthink Pro so that I still have full control over the indexed files in the macOS filesystem, and I can also browse the files in other apps such as the Finder, Terminal, or Leap, creating what Douglas Barone in 2009 called the “file system infobase manager”: “rather than putting data into an [‘everything bucket’] application and using the ho-hum functions of that app to work with my ideas, I keep my data separate and have best-in-class applications, using higher levels of functionality, work on it. […] By using ‘everything bucket’ applications you give up functionality for compactness and eventually that equation works against your creative process. By working in the file system you use the best app for each specific purpose.”