The sample demonstrates how to set PDF page orientation for 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="pageOrientation_PDF.App">
             
    <Application.Resources>
        
    </Application.Resources>
</Application>

using System;
using System.Windows;

namespace pageOrientation_PDF
{
    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 pageOrientation_PDF
	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="pageOrientation_PDF.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">

    <Grid x:Name="LayoutRoot" Background="White" Loaded="LayoutRoot_Loaded">
        <Button Content="RUN" Height="23" HorizontalAlignment="Left" Margin="297,265,0,0" Name="button1" VerticalAlignment="Top" Width="75" Click="button1_Click" />
    </Grid>
</UserControl>    

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

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

namespace pageOrientation_PDF
{
    public partial class MainPage : UserControl
    {
        private SaveFileDialog saveFileDialog = 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 pdfDocument
            PdfDocument document = new PdfDocument();

            //add one portrait page
            PdfSection section1 = document.Sections.Add();
            section1.PageSettings.Orientation = PdfPageOrientation.Portrait;
            section1.PageSettings.Size = PdfPageSize.A4;
            PdfPageBase page = section1.Pages.Add();

            //draw the portrait page
            DrawPage(page);

            //add one landscape page
            PdfSection section2 = document.Sections.Add();
            section2.PageSettings.Orientation = PdfPageOrientation.Landscape;
            section2.PageSettings.Size = PdfPageSize.A4;
            PdfPageBase page21 = section2.Pages.Add();

            //draw the landscape page
            DrawPage(page21);

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

        private void DrawPage(PdfPageBase page)
        {
            float pageWidth = page.Canvas.ClientSize.Width;
            float y = 0;

            //title
            y = y + 5;
            PdfBrush brush1 = new PdfSolidBrush(Color.FromArgb(255, 0, 0, 0));
            PdfFont font1 = new PdfFont(PdfFontFamily.Helvetica, 30f);
            PdfStringFormat format1 = new PdfStringFormat(PdfTextAlignment.Center);
            format1.CharacterSpacing = 1f;
            String text = "Spire.Office";
            page.Canvas.DrawString(text, font1, brush1, pageWidth / 2, y, format1);
            SizeF size = font1.MeasureString(text, format1);
            y = y + size.Height + 6;

            //draw the content
            PdfFont font = new PdfFont(PdfFontFamily.Helvetica, 15f);
            PdfStringFormat format = new PdfStringFormat();
            PdfBrush brush = PdfBrushes.DeepSkyBlue;
            format.LineSpacing = font.Size * 1.5f;
            float x1 = 2*size.Width;
            y = y + size.Height;

            //content
            text = "Spire.Office for .NET is a compilation of every .NET component offered by e-iceblue. It includes Spire.Doc, Spire.XLS, Spire.PDFViewer, Spire.PDF and Spire.DataExport. Spire.Office contains the most up-to-date versions of the components above. Using Spire.Office for .NET developers can create a wide range of applications." +
                   "Consistently offer high-quality, pure and stable .NET components for handling office format files. E-iceblue product lines include components for Word, Excel, PDF and component specially used for data exporting. For E-iceblue, customer service and products quality are the same most important and we guarantee 100% honesty to" +
                   "customers. E-iceblue Welcome any feedback from customers and promise timely and effectively response. E-iceblue .NET component products save your time and save your money from now on.";

            PdfStringLayouter textLayouter = new PdfStringLayouter();
            PdfStringLayoutResult result
                = textLayouter.Layout(text, font, format, new SizeF(pageWidth, 0));

            foreach (LineInfo line in result.Lines)
            {
                page.Canvas.DrawString(line.Text, font, brush, 0, y, format);
                y = y + result.LineHeight;
            }
        }

        private void LayoutRoot_Loaded(object sender, RoutedEventArgs e)
        {

        }
    }
}

Imports System.Windows
Imports System.Windows.Controls
Imports System.Windows.Media
Imports System.IO
Imports System.Drawing

Imports Spire.Pdf
Imports Spire.Pdf.Graphics

Namespace pageOrientation_PDF
	Partial Public Class MainPage
		Inherits UserControl
		Private saveFileDialog As SaveFileDialog = 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 pdfDocument
			Dim document As New PdfDocument()

			'add one portrait page
			Dim section1 As PdfSection = document.Sections.Add()
			section1.PageSettings.Orientation = PdfPageOrientation.Portrait
			section1.PageSettings.Size = PdfPageSize.A4
			Dim page As PdfPageBase = section1.Pages.Add()

			'draw the portrait page
			DrawPage(page)

			'add one landscape page
			Dim section2 As PdfSection = document.Sections.Add()
			section2.PageSettings.Orientation = PdfPageOrientation.Landscape
			section2.PageSettings.Size = PdfPageSize.A4
			Dim page21 As PdfPageBase = section2.Pages.Add()

			'draw the landscape page
			DrawPage(page21)

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

		Private Sub DrawPage(ByVal page As PdfPageBase)
			Dim pageWidth As Single = page.Canvas.ClientSize.Width
			Dim y As Single = 0

			'title
			y = y + 5
			Dim brush1 As PdfBrush = New PdfSolidBrush(Color.FromArgb(255, 0, 0, 0))
			Dim font1 As New PdfFont(PdfFontFamily.Helvetica, 30f)
			Dim format1 As New PdfStringFormat(PdfTextAlignment.Center)
			format1.CharacterSpacing = 1f
			Dim text As String = "Spire.Office"
			page.Canvas.DrawString(text, font1, brush1, pageWidth / 2, y, format1)
			Dim size As SizeF = font1.MeasureString(text, format1)
			y = y + size.Height + 6

			'draw the content
			Dim font As New PdfFont(PdfFontFamily.Helvetica, 15f)
			Dim format As New PdfStringFormat()
			Dim brush As PdfBrush = PdfBrushes.DeepSkyBlue
			format.LineSpacing = font.Size * 1.5f
			Dim x1 As Single = 2*size.Width
			y = y + size.Height

			'content
			text = "Spire.Office for .NET is a compilation of every .NET component offered by e-iceblue. It includes Spire.Doc, Spire.XLS, Spire.PDFViewer, Spire.PDF and Spire.DataExport. Spire.Office contains the most up-to-date versions of the components above. Using Spire.Office for .NET developers can create a wide range of applications." & "Consistently offer high-quality, pure and stable .NET components for handling office format files. E-iceblue product lines include components for Word, Excel, PDF and component specially used for data exporting. For E-iceblue, customer service and products quality are the same most important and we guarantee 100% honesty to" & "customers. E-iceblue Welcome any feedback from customers and promise timely and effectively response. E-iceblue .NET component products save your time and save your money from now on."

			Dim textLayouter As New PdfStringLayouter()
			Dim result As PdfStringLayoutResult = textLayouter.Layout(text, font, format, New SizeF(pageWidth, 0))

			For Each line As LineInfo In result.Lines
				page.Canvas.DrawString(line.Text, font, brush, 0, y, format)
				y = y + result.LineHeight
			Next line
		End Sub

		Private Sub LayoutRoot_Loaded(ByVal sender As Object, ByVal e As RoutedEventArgs)

		End Sub
	End Class
End Namespace