Click the Note tool at Comment > Annotate. Click the note location and enter your comment. The author name will be automatically inserted in the caption of the note. Right-click on the note to start the Properties dialog and change various objects properties. For more details on object properties, see Annotation and Markup Object Properties.
You can reply to notes and work with them.
Click the Comments tool on the left of the View bar to display the Comments panel. Select a note, click the Reply button and enter your text. When you receive a file with notes you need to revise, use the Set Status button to accept, reject it and more. For details, see the Comments panel topic.
If the notes in a document are selected to be flattened, an icon will show the location of the note, but the note text will no longer be available. Notes can be totally removed by Document Inspection if the category Comments is selected.