A good copy editor works toward the same goal as the writer: to serve the reader. I can do that at any of four levels:

Developmental editing: big picture stuff, a review of the overall work, paying attention to content, themes, structure, and more, with an eye toward revision of any of these elements or considering new directions.

Substantive editing: minor to moderate re-writing to eliminate wording that's unclear or confusing and to improve the style, voice, and word choice, while maintaining the author's distinctive voice and style.

Copyediting: improving accuracy, clarity, consistency, and elegance of expression, while correcting grammar or spelling errors and similar flaws.

Proofreading: a thorough review of a document that's already basically finished, checking for typos and errors in grammar, usage, spelling, punctuation, capitalization, abbreviations, and consistency.

I prefer to work in Microsoft Word, using its "Track Changes" feature, so you see exactly what I've done and the difference it makes.

Oh, and I have discovered that I love indexing. Send that work my way, and you'll be pleased with what you get.