It seems to be working in the WindowsForms application but not in an UWP appliction. CAn that be the thing?
I added these lines in the demo project "Syncfusion.SampleBrowser.UWP.Pdf" in the AnnotationFlatten.xaml.cs and there it won't flatten.
See attachment :)
List<float> linePoints = new List<float> { 40, 300, 60, 100, 40, 50, 40, 300 };
//Creates a new ink annotation
RectangleF rectangle = new RectangleF(0, 0, 300, 400);
PdfInkAnnotation inkAnnotation = new PdfInkAnnotation(rectangle, linePoints);
inkAnnotation.Color = new PdfColor(System.Drawing.Color.FromArgb(255, 255, 0, 0));
Attachment:
Pdf_6857024.zip