You might want to hide your comments or markups temporarily for your convenient reading when there are too many annotations on your PDF file. Click the "Hide Annotations" button to hide the markups from view and expose the original page. Click it again to display all the annotations in the document.
After adding annotations to your PDF or opening a PDF file with annotations therein, you can delete the markup or comment by selecting the same in the Comments Panel and clicking the Delete Key, right-clicking it to choose “Delete”, or selecting the "Delete" option in the drop-down menu under the "Edit" button on the Top Menu. You might group select multiple comments and delete them altogether rather than deleting each one individually.