int pageIndex = 0;
PdfCommon.Initialize();
using (var doc = PdfDocument.CreateNew())
{
    var files = System.IO.Directory.GetFiles(@"SourceImages", "*.*", System.IO.SearchOption.AllDirectories);
    foreach (var file in files)
    {
        using (var image = Bitmap.FromFile(file, true) as Bitmap)
        {
		var pdfBitmap = CreateBitmap(image);
		var imageObject = PdfImageObject.Create(doc);
		imageObject.SetBitmap(pdfBitmap);
	 
		var size = CalculateSize(pdfBitmap.Width, pdfBitmap.Height, image.HorizontalResolution, image.VerticalResolution);

		doc.Pages.InsertPageAt(pageIndex, size);
		doc.Pages[pageIndex].PageObjects.InsertObject(imageObject);
 
		imageObject.SetMatrix(size.Width, 0, 0, size.Height, 0, 0);
 
		doc.Pages[pageIndex].GenerateContent();
		pageIndex++;
        }
    }
    doc.Save(@"saved.pdf", SaveFlags.NoIncremental);
}

private static PdfBitmap CreateBitmap(Bitmap image)
{
    BitmapFormats pdfFormat;
    int[] palette;
    GetPdfFormat(image, out pdfFormat, out palette);
 
    var lockInfo = image.LockBits(new Rectangle(0, 0, image.Width, image.Height), ImageLockMode.ReadOnly, image.PixelFormat);
    var pdfBitmap = new PdfBitmap(image.Width, image.Height, pdfFormat, lockInfo.Scan0, lockInfo.Stride);
    image.UnlockBits(lockInfo);
 
    if (palette != null)
	pdfBitmap.Palette = palette;
    return pdfBitmap;
}
 
public static void GetPdfFormat(Bitmap image, out BitmapFormats pdfFormat, out int[] palette)
{
    palette = null;
    switch (image.PixelFormat)
    {
	case PixelFormat.Format1bppIndexed:
	    pdfFormat = BitmapFormats.FXDIB_1bppRgb;
	    palette = GetPalette(image);
	    break;
	case PixelFormat.Format8bppIndexed:
	    pdfFormat = BitmapFormats.FXDIB_8bppRgb;
	    palette = GetPalette(image);
	    break;
	case PixelFormat.Format24bppRgb:
	    pdfFormat = BitmapFormats.FXDIB_Rgb;
	    break;
	case PixelFormat.Format32bppArgb:
	case PixelFormat.Format32bppPArgb:
	    pdfFormat = BitmapFormats.FXDIB_Argb;
	    break;
	case PixelFormat.Format32bppRgb:
	    pdfFormat = BitmapFormats.FXDIB_Rgb32;
	    break;
	default:
	    throw new Exception("Unsupported Image Format");
	}
    }
 
public static int[] GetPalette(Bitmap image)
{
    var ret = new int[image.Palette.Entries.Length];
    for (int i = 0; i < ret.Length; i++)
	ret[i] = ((Color)image.Palette.Entries.GetValue(i)).ToArgb();
    return ret;
}
 
private static SizeF CalculateSize(int width, int height, float dpiX, float dpiY)
{
    return new SizeF()
    {
	Width = width * 72 / dpiX,
	Height = height * 72 / dpiY
    };
}