Get All Annotations from Page in a PDF
To get all annotations from a page in a PDF document, loop through a Page object’s AnnotationCollection collection.
The following code snippet shows you how to get all the annotations of a page.
Please note that to get all the annotations from the whole PDF, you have to loop through the document’s PageCollection collection before looping through the Annotations collection for each page. You can get each annotation of the collection in a base annotation type called MarkupAnnotation and then show its properties.