This article shows how to get annotation from the pdf.

All these methods can be invoked after ObjsStart or Render or RenderToBmp.
All these methods are valid in professional or premium version.

You have three ways to get annotations:

page.GetAnnotByName(String name);
page.GetAnnot(int index);
page.GetAnnotFromPoint(int x, int y);

To get the last annot you have added you can use something like that:

page.GetAnnot(page.GetAnnotCount() - 1 );