The following annotations are available in the Comment tab.

Note: This is like a sticky note, and can be glued on top of the document page.

Text Box: Enter text phrases directly on pages with selectable font, style, color, and transparency. Callouts are special text boxes pointing to the place that a comment is made on.

Drawing: Enter graphic objects, or even a stamp, directly on pages with selectable shape, color, and other property settings.

Markup: Emphasize text parts on pages with highlight, cross-out, or underline. Also indicate suggested additions, deletions or changes to a document.

Attachment: Attach files, sounds, movies to the document.

Comment tools like Note, Text Box, Draw, Markup, and Attach File share many common properties. For details, see Annotation and Markup Object Properties.

Under security settings you can allow or prohibit commenting.

When you have reviewed a comment, select "Set Status" from its shortcut menu to change its "Review status" and/or Migrate status. See Comments panel for details.

Annotations are stored in the PDF at a different level to the real PDF content. They allow shared document commenting. See also Print with Comments.