This is such a specialized app, almost a feature to a larger app, that it's hard to even imagine a target for it. Did you stumble upon a problem in the wild that your app solves? Without a clear understanding of a target, you can't do marketing (and that's assuming there's a market for this). Even something basic like a persona could help you market this app better.

What about creating an addon for Google Docs that integrates with your app? Maybe a freemium offering can help reach customers?

Have you considered selling it on Flippa?