Annotation flattening refers to the operation that changes annotations into a static area that is part of the PDF document, just like the other text and images in the document. When flattening an annotation, the annotation is removed from the document, while its visual representation is kept intact. A flattened annotation is visible but is non-editable by your users or by your app.
Annotations in a PDF document can be flattened in the ComPDFKit by saving the document and choosing the Flatten mode.
CPDFDocument document = CPDFDocument.InitWithFilePath("filePath");
document.WriteFlattenToFilePath("savePath");