PdfDocument document = new PdfDocument();
document.LoadFromFile(@"sample.pdf");

  private static Image[] SaveAsImage(PdfDocument document)
          {
            Image[] images = new Image[document.Pages.Count];
            for (int i = 0; i < document.Pages.Count; i++)
              {
//use the document.SaveAsImage() method save the pdf as image
                images[i] = document.SaveAsImage(i);
           }
            return images;
          }

   public static void JoinTiffImages(Image[] images, string outFile, EncoderValue compressEncoder)
        {
            //use the save encoder
            Encoder enc = Encoder.SaveFlag;
            EncoderParameters ep = new EncoderParameters(2);
            ep.Param[0] = new EncoderParameter(enc, (long)EncoderValue.MultiFrame);
            ep.Param[1] = new EncoderParameter(Encoder.Compression, (long)compressEncoder);
            Image pages = images[0];
            int frame = 0;
            ImageCodecInfo info = GetEncoderInfo("image/tiff");
            foreach (Image img in images)
            {
                if (frame == 0)
                {
                    pages = img;
                    //save the first frame
                    pages.Save(outFile, info, ep);
                }
                else
                {
                    //save the intermediate frames
                    ep.Param[0] = new EncoderParameter(enc, (long)EncoderValue.FrameDimensionPage);
                    pages.SaveAdd(img, ep);
               }
                if (frame == images.Length - 1)
                {
                    //flush and close.
                    ep.Param[0] = new EncoderParameter(enc, (long)EncoderValue.Flush);
                    pages.SaveAdd(ep);
                }
                frame++;
            }
        }

using System;
using System.Drawing;
using System.Drawing.Imaging;
using Spire.Pdf;
namespace SavePdfAsTiff
{
    class Program
    {
        static void Main(string[] args)
        {
            PdfDocument document = new PdfDocument();
            document.LoadFromFile(@"01.pdf");
    JoinTiffImages(SaveAsImage(document),"result.tiff",EncoderValue.CompressionLZW);
            System.Diagnostics.Process.Start("result.tiff");
        }
        private static Image[] SaveAsImage(PdfDocument document)
        {
            Image[] images = new Image[document.Pages.Count];
            for (int i = 0; i < document.Pages.Count; i++)
            {
                images[i] = document.SaveAsImage(i);
            }
            return images;
        }

        private static ImageCodecInfo GetEncoderInfo(string mimeType)
        {
            ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();
            for (int j = 0; j < encoders.Length; j++)
            {
                if (encoders[j].MimeType == mimeType)
                    return encoders[j];
            }
            throw new Exception(mimeType + " mime type not found in ImageCodecInfo");
        }

        public static void JoinTiffImages(Image[] images, string outFile, EncoderValue compressEncoder)
        {
            //use the save encoder
            Encoder enc = Encoder.SaveFlag;
            EncoderParameters ep = new EncoderParameters(2);
            ep.Param[0] = new EncoderParameter(enc, (long)EncoderValue.MultiFrame);
            ep.Param[1] = new EncoderParameter(Encoder.Compression, (long)compressEncoder);
            Image pages = images[0];
            int frame = 0;
            ImageCodecInfo info = GetEncoderInfo("image/tiff");
            foreach (Image img in images)
            {
                if (frame == 0)
                {
                    pages = img;
                    //save the first frame
                    pages.Save(outFile, info, ep);
                }

                else
                {
                    //save the intermediate frames
                    ep.Param[0] = new EncoderParameter(enc, (long)EncoderValue.FrameDimensionPage);

                    pages.SaveAdd(img, ep);
                }
                if (frame == images.Length - 1)
                {
                    //flush and close.
                    ep.Param[0] = new EncoderParameter(enc, (long)EncoderValue.Flush);
                    pages.SaveAdd(ep);
                }
                frame++;
            }
        }
    }
}

Imports System
Imports System.Drawing
Imports System.Drawing.Imaging
Imports Spire.Pdf
Namespace SavePdfAsTiff
    Class Program
        Sub Main()
            Dim document As PdfDocument = New PdfDocument()
            document.LoadFromFile("01.pdf")
            JoinTiffImages(SaveAsImage(document), "result.tiff", EncoderValue.CompressionLZW)
            System.Diagnostics.Process.Start("result.tiff")
        End Sub
        Private Shared Function SaveAsImage(ByVal document As PdfDocument)
            Dim images() As Image = New Image(document.Pages.Count-1) {}
            Dim i As Integer
            For i = 0 To document.Pages.Count - 1 Step i + 1
                images(i) = document.SaveAsImage(i)
            Next
            Return images
        End Function
        Private Shared Function GetEncoderInfo(ByVal mimeType As String) As ImageCodecInfo
            Dim encoders() As ImageCodecInfo = ImageCodecInfo.GetImageEncoders()
            Dim j As Integer
            For j = 0 To encoders.Length - 1 Step j + 1
                If encoders(j).MimeType = mimeType Then
                    Return encoders(j)
                End If
            Next
            Throw New Exception(mimeType + " mime type not found in ImageCodecInfo")
        End Function
        Public Shared Sub JoinTiffImages(ByVal images() As Image, ByVal outFile As String, ByVal compressEncoder As EncoderValue)
            'use the save encoder
            Dim enc As Encoder = Encoder.SaveFlag
            Dim ep As EncoderParameters = New EncoderParameters(2)
            ep.Param(0) = New EncoderParameter(enc, CType(EncoderValue.MultiFrame, Long))
            ep.Param(1) = New EncoderParameter(Encoder.Compression, CType(compressEncoder, Long))
            Dim pages As Image = images(0)
            Dim frame As Integer = 0
            Dim info As ImageCodecInfo = GetEncoderInfo("image/tiff")
            Dim img As Image
            For Each img In images
                If frame = 0 Then
                    pages = img
                    'save the first frame
                    pages.Save(outFile, info, ep)
                Else
                    'save the intermediate frames
                    ep.Param(0) = New EncoderParameter(enc, CType(EncoderValue.FrameDimensionPage, Long))
                    pages.SaveAdd(img, ep)
                End If

                If frame = images.Length - 1 Then
                    'flush and close.
                    ep.Param(0) = New EncoderParameter(enc, CType(EncoderValue.Flush, Long))
                    pages.SaveAdd(ep)
                End If
                frame = frame + 1
            Next
        End Sub
    End Class
End Namespace

public Image SaveAsImage(int pageIndex, int dpiX, int dpiY)

Image image = documemt.SaveAsImage(0, PdfImageType.Bitmap, false, 400, 400)

PdfDocument documemt = new PdfDocument();
documemt.LoadFromFile(@"..\..\EnglishText.pdf");
Image image = documemt.SaveAsImage(0, PdfImageType.Bitmap, false, 400, 400);
image.Save(@"..\..\result.jpg");
documemt.Close();