Adding a 3D comment into a 3D viewport can be performed by attaching the comment to a view.
It is often a good practice to create a special view to hold the desired orientation and comment.
Only when this view is selected is the 3D comment visible.
The PDF can be viewed, commented saved and reloaded by someone else.
The order of operations is:
a) Create a 3D PDF without security - to allow commenting, editing and saving
b) Open model tree menu on the left side
c) Add a new camera view, name it to something meaningful
d) In Adobe Reader, right-mouse click in 3D view, expose tools sub-menu, click Add 3D Comment
e) In Acrobat, Using 3D toolbar - click Add a 3D comment button, select 3D position and message
f) Save the edited PDF
g) Reload in a new session, go to new view to see the 3D comment