The sample demonstrates how to make PDF overlay effect in Silverlight via Spire.PDF.

<Application xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
             x:Class="Overlay.App">
             
    <Application.Resources>
        
    </Application.Resources>
</Application>

using System;
using System.Windows;

namespace Overlay
{
    public partial class App : Application
    {

        public App()
        {
            this.Startup += this.Application_Startup;
            this.Exit += this.Application_Exit;
            this.UnhandledException += this.Application_UnhandledException;

            InitializeComponent();
        }

        private void Application_Startup(object sender, StartupEventArgs e)
        {
            this.RootVisual = new MainPage();
        }

        private void Application_Exit(object sender, EventArgs e)
        {

        }

        private void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
        {
            // If the app is running outside of the debugger then report the exception using
            // the browser's exception mechanism. On IE this will display it a yellow alert 
            // icon in the status bar and Firefox will display a script error.
            if (!System.Diagnostics.Debugger.IsAttached)
            {

                // NOTE: This will allow the application to continue running after an exception has been thrown
                // but not handled. 
                // For production applications this error handling should be replaced with something that will 
                // report the error to the website and stop the application.
                e.Handled = true;
                Deployment.Current.Dispatcher.BeginInvoke(delegate { ReportErrorToDOM(e); });
            }
        }

        private void ReportErrorToDOM(ApplicationUnhandledExceptionEventArgs e)
        {
            try
            {
                string errorMsg = e.ExceptionObject.Message + e.ExceptionObject.StackTrace;
                errorMsg = errorMsg.Replace('"', '\'').Replace("\r\n", @"\n");

                System.Windows.Browser.HtmlPage.Window.Eval("throw new Error(\"Unhandled Error in Silverlight Application " + errorMsg + "\");");
            }
            catch (Exception)
            {
            }
        }
    }
}

Imports System.Windows

Namespace Overlay
	Partial Public Class App
		Inherits Application

		Public Sub New()
			AddHandler Me.Startup, AddressOf Application_Startup
			AddHandler Me.Exit, AddressOf Application_Exit
			AddHandler Me.UnhandledException, AddressOf Application_UnhandledException

			InitializeComponent()
		End Sub

		Private Sub Application_Startup(ByVal sender As Object, ByVal e As StartupEventArgs)
			Me.RootVisual = New MainPage()
		End Sub

		Private Sub Application_Exit(ByVal sender As Object, ByVal e As EventArgs)

		End Sub

		Private Sub Application_UnhandledException(ByVal sender As Object, ByVal e As ApplicationUnhandledExceptionEventArgs)
			' If the app is running outside of the debugger then report the exception using
			' the browser's exception mechanism. On IE this will display it a yellow alert 
			' icon in the status bar and Firefox will display a script error.
			If Not Debugger.IsAttached Then

				' NOTE: This will allow the application to continue running after an exception has been thrown
				' but not handled. 
				' For production applications this error handling should be replaced with something that will 
				' report the error to the website and stop the application.
				e.Handled = True
				Deployment.Current.Dispatcher.BeginInvoke(Sub() ReportErrorToDOM(e))
			End If
		End Sub

		Private Sub ReportErrorToDOM(ByVal e As ApplicationUnhandledExceptionEventArgs)
			Try
				Dim errorMsg As String = e.ExceptionObject.Message + e.ExceptionObject.StackTrace
				errorMsg = errorMsg.Replace(""""c, "'"c).Replace(vbCrLf, vbLf)

				System.Windows.Browser.HtmlPage.Window.Eval("throw new Error(""Unhandled Error in Silverlight Application " & errorMsg & """);")
			Catch e1 As Exception
			End Try
		End Sub
	End Class
End Namespace

<UserControl x:Class="Overlay.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400" Loaded="UserControl_Loaded">

    <Grid x:Name="LayoutRoot" Background="White">
        <Button Content="Run" Height="23" HorizontalAlignment="Left" Margin="295,254,0,0" Name="button1" VerticalAlignment="Top" Width="75" Click="button1_Click" />
    </Grid>
</UserControl>    

using System.Windows;
using System.Windows.Controls;
using System.IO;
using System.Reflection;
using System.Drawing;

using Spire.Pdf;
using Spire.Pdf.Graphics;

namespace Overlay
{
    public partial class MainPage : UserControl
    {
        private SaveFileDialog saveFileDialog = null;
        private PdfDocument pdfDoc = null;
        public MainPage()
        {
            InitializeComponent();
            this.saveFileDialog = new SaveFileDialog();
            this.saveFileDialog.Filter = "PDF Document(*.pdf)|*.pdf";
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            //create a page template
            PdfTemplate template = this.pdfDoc.Pages[3].CreateTemplate();
            PointF location = new PointF(20, -40);          
            //overlay the pdfDoc
            foreach (PdfPageBase page in this.pdfDoc.Pages)
            {
                page.Canvas.SetTransparency(0.25f, 0.25f, PdfBlendMode.Overlay);
                page.Canvas.DrawTemplate(template, location);
            }

            //save the pdfDoc
            bool? result = this.saveFileDialog.ShowDialog();
            if (result.HasValue && result.Value)
            {
                using (Stream stream = this.saveFileDialog.OpenFile())
                {
                    this.pdfDoc.SaveToStream(stream);
                }
            }
        }

        private void UserControl_Loaded(object sender, RoutedEventArgs e)
        {
            //load the pdf templates
            Assembly assembly = this.GetType().Assembly;

            foreach (string name in assembly.GetManifestResourceNames())
            {
                if (name.EndsWith("Sample3.pdf"))
                {
                    this.pdfDoc = new PdfDocument(assembly.GetManifestResourceStream(name));
                }
            }
        }
    }
}

Imports System.Windows
Imports System.Windows.Controls
Imports System.IO
Imports System.Reflection
Imports System.Drawing

Imports Spire.Pdf
Imports Spire.Pdf.Graphics

Namespace Overlay
	Partial Public Class MainPage
		Inherits UserControl
		Private saveFileDialog As SaveFileDialog = Nothing
		Private pdfDoc As PdfDocument = Nothing
		Public Sub New()
			InitializeComponent()
			Me.saveFileDialog = New SaveFileDialog()
			Me.saveFileDialog.Filter = "PDF Document(*.pdf)|*.pdf"
		End Sub

		Private Sub button1_Click(ByVal sender As Object, ByVal e As RoutedEventArgs)
			'create a page template
			Dim template As PdfTemplate = Me.pdfDoc.Pages(3).CreateTemplate()
                        Dim location As PointF=New PointF(20,-40);
                        'overlay the pdfDoc
			For Each page As PdfPageBase In Me.pdfDoc.Pages
				page.Canvas.SetTransparency(0.25f, 0.25f, PdfBlendMode.Overlay)
				page.Canvas.DrawTemplate(template, location)
			Next page

			'save the pdfDoc
			Dim result? As Boolean = Me.saveFileDialog.ShowDialog()
			If result.HasValue AndAlso result.Value Then
				Using stream As Stream = Me.saveFileDialog.OpenFile()
					Me.pdfDoc.SaveToStream(stream)
				End Using
			End If
		End Sub

		Private Sub UserControl_Loaded(ByVal sender As Object, ByVal e As RoutedEventArgs)
			'load the pdf templates
			Dim [assembly] As System.Reflection.Assembly = Me.GetType().Assembly

			For Each name As String In [assembly].GetManifestResourceNames()
				If name.EndsWith("Sample3.pdf") Then
					Me.pdfDoc = New PdfDocument([assembly].GetManifestResourceStream(name))
				End If
			Next name
		End Sub
	End Class
End Namespace