PM> Install-Package Spire.PDF

using System;
using System.Drawing;
using System.Drawing.Imaging;
using Spire.Pdf;

namespace SavePdfAsTiff
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a PdfDocument object
            PdfDocument pdf = new PdfDocument();

            //Load a PDF document
            pdf.LoadFromFile("Report.pdf");

            //Convert PDF pages to images
            Image[] images = SaveAsImage(pdf);

            //Combine the images and save them as a multi-page TIFF file
            JoinTiffImages(images, "result.tiff", EncoderValue.CompressionLZW);
        }

        private static Image[] SaveAsImage(PdfDocument document)
        {
            //Create a new image array
            Image[] images = new Image[document.Pages.Count];

            //Iterate through all pages in the document
            for (int i = 0; i < document.Pages.Count; i++)
            {
                //Convert a specific page to an image
                images[i] = document.SaveAsImage(i);
            }
            return images;
        }

        private static ImageCodecInfo GetEncoderInfo(string mimeType)
        {
            //Get the image encoders
            ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();
            for (int j = 0; j < encoders.Length; j++)
            {
                //Find the encoder that matches the specified MIME type
                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)
        {
            //Set the encoder parameters 
            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);

            //Get the first image
            Image pages = images[0];
            //Initialize a frame
            int frame = 0;
            //Get an ImageCodecInfo object for processing TIFF image codec information
            ImageCodecInfo info = GetEncoderInfo("image/tiff");

            //Iterate through each Image
            foreach (Image img in images)
            {
                //If it's the first frame, save it to the output file with specified encoder parameters
                if (frame == 0)
                {
                    pages = img;
                    pages.Save(outFile, info, ep);
                }

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

                    pages.SaveAdd(img, ep);
                }
                //If it's the last frame, flush the encoder parameters and close the file
                if (frame == images.Length - 1)
                {
                    ep.Param[0] = new EncoderParameter(enc, (long)EncoderValue.Flush);
                    pages.SaveAdd(ep);
                }
                frame++;
            }
        }
    }
}

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using Spire.Pdf;
using Spire.Pdf.Graphics;

namespace TiffToPdf
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a PdfDocument object
            PdfDocument pdf = new PdfDocument();

            //Load a TIFF image
            Image tiffImage = Image.FromFile("result.tiff");

            //Split the Tiff image into separate images
            Image[] images = SplitTiffImage(tiffImage);

            //Iterate through the images
            for (int i = 0; i < images.Length; i++)
            {
                //Convert a specified image into a PDF image
                PdfImage pdfImg = PdfImage.FromImage(images[i]);

                //Get image width and height
                float width = pdfImg.Width;
                float height = pdfImg.Height;

                //Add a page with the same size as the image
                SizeF size = new SizeF(width, height);
                PdfPageBase page = pdf.Pages.Add(size);

                //Draw the image at a specified location on the page
                page.Canvas.DrawImage(pdfImg, 0, 0, width, height);
            }

            //Save the result file
            pdf.SaveToFile("TiffToPdf.pdf");
        }

        public static Image[] SplitTiffImage(Image tiffImage)
        {
            //Get the number of frames in the Tiff image
            int frameCount = tiffImage.GetFrameCount(FrameDimension.Page);
            //Create an image array to store the split tiff images
            Image[] images = new Image[frameCount];
            //Gets the GUID of the first frame dimension
            Guid objGuid = tiffImage.FrameDimensionsList[0];
            //Create a FrameDimension object
            FrameDimension objDimension = new FrameDimension(objGuid);

            //Iterate through each frame
            for (int i = 0; i < frameCount; i++)
            {
                //Select a specified frame
                tiffImage.SelectActiveFrame(objDimension, i);
                //Save the frame in TIFF format to a memory stream
                MemoryStream ms = new MemoryStream();
                tiffImage.Save(ms, ImageFormat.Tiff);
                //Load an image from memory stream
                images[i] = Image.FromStream(ms);
            }
            return images;
        }
    }
}