string pastapdf = AppDomain.CurrentDomain.BaseDirectory + "temporario\" + Path.GetFileNameWithoutExtension(e[i]);
Directory.CreateDirectory(pastapdf);
string pdfFile = Path.Combine(pastapdf, e[i]);
for (int h = 0; h <= (pdfReader.NumberOfPages - 1); h++)
{
    using (var document = PdfiumViewer.PdfDocument.Load(pdfFile))
    {
         var image = document.Render(h, dpi, dpi, true);
         image.Save(pastapdf + "\" + Path.GetFileNameWithoutExtension(e[i]) + "_" + h.ToString() + ".tif", ImageFormat.Tiff);
         image.Dispose();
    }
}
string save = output + "\" + Path.GetFileNameWithoutExtension(e[i]) + ".tif";
string[] tifffiles = GetFiles(pastapdf, "*.tif");
var finalfiles = tifffiles.OrderBy(f =>
    new string(f.Where(char.IsLetter).ToArray())).ThenBy(f => {
    int number; if (int.TryParse(new string(f.Where(char.IsDigit).ToArray()), out number))
    return number; return -1; } ).ToList(); //código LINQ para ordenar

JoinTiffLZW(finalfiles.ToArray(), save);
Invoke(new Action(() => { pbar.PerformStep(); pdfReader.Dispose(); }));