Saved comments and annotations can be found in the My Comments section of the My Comments Toolchest.
To save an annotation, right-click (Mac: Ctrl+click) on it to show the context menu and select Add to My Comments from the menu.
To add a saved annotation to a document, click the annotation in My Comments, and then click the location on the document to place the annotation. Once placed the annotation can be edited as normal.
Saved comments can be reordered, deleted, or resized.