PdfWatermarkOpacity Property Terminal Works - PDF Printing.Net Library
Opacity value in percentage - 0 is invisible and at 100 isn't transparent at all.

Namespace: PdfPrintingNet
Assembly: PdfPrintingNet (in PdfPrintingNet.dll) Version: 5.1.1.0 (5.1.1.0)
Syntax

public float Opacity { get; set; }

Property Value

Type: Single
See Also

Reference