AddingEllipseAnnotationInPDF.cs

// For complete examples and data files, please go to https://github.com/groupdocs-annotation/GroupDocs.Annotation-for-.NET
// Get input file stream
Stream inputFile = new FileStream("E:\sample.pdf", FileMode.Open, FileAccess.ReadWrite);
// Initialize list of AnnotationInfo
List<AnnotationInfo> annotations = new List<AnnotationInfo>();
// Initialize ellipse annotation.
AnnotationInfo ellipse = new AnnotationInfo
{
    Box = new Rectangle(100, 100, 50, 50),
    Type = AnnotationType.Ellipse;
};
// Add annotation to list
annotations.Add(ellipse);
// Export annotation and save output file
CommonUtilities.SaveOutputDocument(inputFile, annotations, DocumentType.Pdf);