Comments can be applied to lines of text in the document, specifically, as annotations. Select the text you’d like to annotate and then click the annotation icon, on the left.
Enter your feedback and click the Add Annotation button. The text you’ve annotated will be highlighted with a yellow background and will show your annotation whenever another user rolls over the highlighted text. Text with multiple annotations will be highlighted in a brighter, deeper yellow. Annotations can only be left by collaborators with Editor or Co-owner access levels.
You can also leave a comment on the document, by using the comment button from the Files screen.
When hovering over an existing annotation, you can click Add Reply to respond to an existing annotation within the same text range. This action does not send an email notification, it appends additional text to the existing annotation.