You also need this if you are attempting to populate PDF form fields, you also need to set the NeedsAppearances element to true. Otherwise the PDF will “hide” the values on the form. Here is the VB code.
If objPdfSharpDocument.AcroForm.Elements.ContainsKey("/NeedAppearances") = False Then
objPdfSharpDocument.AcroForm.Elements.Add("/NeedAppearances", New PdfSharp.Pdf.PdfBoolean(True))
Else
objPdfSharpDocument.AcroForm.Elements("/NeedAppearances") = New PdfSharp.Pdf.PdfBoolean(True)
End If