ABCpdf .NET PDF Component Documentation - XColor Alpha Property. The alpha opacity.

Here we create a PDF document showing how different values of alpha result in different levels of transparency.

using (Doc doc = new Doc()) {
  doc.Rect.Inset(20, 20);
  doc.FontSize = 300;
  for (int i = 1; i <= 10; i++) {
    doc.Color.Alpha = 255 / i;
    doc.AddText(doc.Color.Alpha.ToString());
    doc.Rect.Move(25, -50);
  }
  doc.Save(Server.MapPath("coloralpha.pdf"));
}

Using doc As New Doc()
  doc.Rect.Inset(20, 20)
  doc.FontSize = 300
  Dim i As Integer = 1
  While i <= 10
    doc.Color.Alpha = 255 / i
    doc.AddText(doc.Color.Alpha.ToString())
    doc.Rect.Move(25, -50)
    System.Math.Max(System.Threading.Interlocked.Increment(i),i - 1)
  End While
  doc.Save(Server.MapPath("coloralpha.pdf"))
End Using