New Post: Single and Multipage TIFFs to SINGLE PDF

using (MagickImageCollection output = new MagickImageCollection())
{
  foreach (string fileName in Directory.GetFiles(@"C:\InputFolder", "*.tiff"))
  {
    using (MagickImageCollection pages = new MagickImageCollection(fileName))
    {
      output.AddRange(pages);
    }
  }
  output.Write("output.pdf");
}