To delete annotations individually:
Tap to activate an existing annotation then tap the trashcan icon in the ribbon that appears at the upper-right of your screen.
To delete annotations in bulk:
You can remove all markups from a document or page by using the Delete All Annotations or Delete Annotations On Page tools, respectively. These tools can be found in the Toolbox under the Document heading.
You can also delete all annotations from a document or page via the Navigation Panel at the left of your screen:
To delete all annotations from the whole document, tap Actions > More Actions > Delete Annotations.
To delete all annotations from a specific page, tap Thumbnails > tap and hold on the desired page > select "Delete Annotations" from the popup menu.
Please note: deleted annotations are not stored anywhere in the app and will not be recoverable.