Export annotations in UWP

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 documentFields = doc.FDFExtract(PDFDocExtractFlag.e_annots_only); //PDFDocExtractFlag.e_forms_only

// Export annotations from FDF to XFDF.
await documentFields.SaveAsXFDFAsync(outputXfdfFileName);

// Optionally write XFDF to a string
String xfdfString = documentFields.SaveAsXFDFString();

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)