How to add text watermarks into existing PDF document
This example demonstrates how to add backround semi-transparent text watermarks into existing PDF documents
output PDF document with semitransparent text watermarks added
Program.vb:
VB

'*******************************************************************
'       ByteScout PDF SDK
'                                                                   
'       Copyright © 2016 Bytescout, http://www.bytescout.com        
'       ALL RIGHTS RESERVED                                         
'                                                                   
'*******************************************************************

Imports Bytescout.PDF

''' <summary>
''' This example demonstrates how to draw transparent objects.
''' </summary>
Class Program

    Shared Sub Main()

        ' Create new document
        Dim pdfDocument = New Document()
        pdfDocument.RegistrationName = "demo"
		pdfDocument.RegistrationKey = "demo"
        ' Add page
        Dim page = New Page(PaperFormat.A4)
        pdfDocument.Pages.Add(page)

        Dim canvas = page.Canvas

        ' Prepare pens and brushes
        Dim borderPen = New SolidPen(New ColorRGB(0, 0, 0), 2.0F)

        Dim brush1 = New SolidBrush(New ColorRGB(255, 0, 0))
        Dim brush2 = New SolidBrush(New ColorRGB(0, 0, 255))

        borderPen.Opacity = 50
        brush1.Opacity = 30
        brush2.Opacity = 60

        ' Draw normal rectangles
        canvas.DrawRectangle(brush1, 100, 100, 100, 100)
        canvas.DrawRectangle(borderPen, brush2, 150, 150, 100, 100)

        borderPen.Opacity = 80
        brush1.Opacity = 60
        brush2.Opacity = 30

        ' Draw rounded rectangles
        canvas.DrawRoundedRectangle(brush1, 220, 100, 100, 100, 10)
        canvas.DrawRoundedRectangle(borderPen, brush2, 270, 150, 100, 100, 10)

        ' Save document to file
        pdfDocument.Save("result.pdf")

        ' Cleanup 
		pdfDocument.Dispose()

        ' Open document in default PDF viewer app
        Process.Start("result.pdf")

    End Sub

End Class 

Program.cs:
C#

//*******************************************************************
//       ByteScout PDF SDK		                                     
//                                                                   
//       Copyright © 2016 ByteScout - http://www.bytescout.com       
//       ALL RIGHTS RESERVED                                         
//                                                                   
//*******************************************************************

using System.Diagnostics;
using Bytescout.PDF;

namespace Transparency
{
	/// <summary>
	/// This example demonstrates how to draw transparent objects.
	/// </summary>
	class Program
	{
		static void Main()
		{
			// Create new document
			Document pdfDocument = new Document();
			pdfDocument.RegistrationName = "demo";
			pdfDocument.RegistrationKey = "demo";
			// Add page
			Page page = new Page(PaperFormat.A4);
			pdfDocument.Pages.Add(page);

			Canvas canvas = page.Canvas;

			// Prepare pens and brushes
			Pen borderPen = new SolidPen(new ColorRGB(0, 0, 0), 2f);
			
			SolidBrush brush1 = new SolidBrush(new ColorRGB(255, 0, 0));
			SolidBrush brush2 = new SolidBrush(new ColorRGB(0, 0, 255));
			
			borderPen.Opacity = 50;
			brush1.Opacity = 30;
			brush2.Opacity = 60;

			// Draw normal rectangles
			canvas.DrawRectangle(brush1, 100, 100, 100, 100);
			canvas.DrawRectangle(borderPen, brush2, 150, 150, 100, 100);

			borderPen.Opacity = 80;
			brush1.Opacity = 60;
			brush2.Opacity = 30;

			// Draw rounded rectangles
			canvas.DrawRoundedRectangle(brush1, 220, 100, 100, 100, 10);
			canvas.DrawRoundedRectangle(borderPen, brush2, 270, 150, 100, 100, 10);
			
			// Save document to file
			pdfDocument.Save("result.pdf");

			// Cleanup 
			pdfDocument.Dispose();

			// Open document in default PDF viewer app
			Process.Start("result.pdf");
		}
	}
}