Do it in track, if it's just a formatting error. However, if you believe there are drafting mistakes, feel free to add comments to highlight the specific issues. I have worked with certain associates who prefer listing the required changes in a notebook and discussing them individually. Regardless, it would be best to ask about the associate's preferred method.

Ask the associate who has assigned you the work! :) Everyone has a different style of working... If you can't ask the question for any reason, the make the changes in track if you're correcting typos, cross-referencing, brackets etc. and add bubble comments if you have more substantial comments (eg. if you feel a sentence needs to be re-drafted or if page numbering is wrong) and re-send the doc to the junior associate