A wide range of drawings can be added to PDF files as comments.
Click this tool in the Annnotate group and select a drawing object from the drop-down list. The topic Drawing tools lists all objects; it tells how each drawing object works and shows a picture of a typical result. Each drawing object is placed along with a note which can be used to add further information related to the drawing object. A typical use for this is to circle a diagram and add text: “make this bigger”.
All drawing objects are treated as comments, so they appear in the Comments Panel and can be sorted, filtered, searched, summarized, replied to, accept a status and more.
A Stamp is also treated as a Comment; it can be considered a predefined drawing object.