Click the down arrow next to Annotate button in the Toolbar to reveal the tools.
Select the Comments tool .
Click in the page location where you’d like to place a comment. You can also click and drag initially in order to size the comment when you place it.
Once your comment is placed, just type over the text that appears.
With either the Edit Tool or the Select Text Tool as the current tool, you can double-click a comment to edit it.
Tips on Adding Notes, Comments, Clouds, or Callouts
You can double-click a tool to lock it on and use it continuously. This is helpful when adding multiple notes and/or comments. Click the Edit tool to unlock your tool (Command+2). To turn the automatic continuous use feature on or off, see Preferences for details.
Use the Object Properties menu to adjust the color and styling of your comment text box. (Object Properties).