A Notes Summary is a textual list of the markups added to any given document that can be included in the body of an email, either with or without the marked-up PDF attached. The type of annotation, color, and page number it appears on, along with any comments added to the annotation, will be included.
Contents of highlighted/underlined/struck-out text is auto-populated in a comment bubble, however, so the highlighted/underlined/struck-out text may appear twice in the Notes Summary if you have not modified the comment bubbles.
In order to avoid your text being duplicated in the Notes Summary, turn off the setting under App Settings > Annotation entitled "Auto-Add Markup to Annotations."
This will prevent the highlighted or underlined content from being copied into the associated comment.
Please note: Turning this option off will not delete markup that as already been added by this switch.