Export annotations in Xamarin

To extract data from PDF to FDF, then export FDF as XFDF

PDFDoc doc = new PDFDoc(filename);

// Extract annotations to FDF.
// Optionally use e_both to extract both forms and annotations
FDFDoc doc_fields = doc.FDFExtract(PDFDoc.ExtractFlag.e_annots_only); //PDFDoc.ExtractFlag.e_forms_only

// Export annotations from FDF to XFDF.
doc_fields.SaveAsXFDF(output_xfdf_filename);

// Optionally write XFDF to a string
string XFDF_string = doc_fields.SaveAsXFDF();

PDF Form Fill and Form Data Extraction
Full sample code which illustrates basic FDF merge/extract functionality and full support for FDF (Forms Data Format)