var filenameBase = Path.GetFileNameWithoutExtension(inputFile);
var pageRanges = new List<PageRange>() {
  new PageRange() { LowerBound = 1, UpperBound = 2 },
  new PageRange() { LowerBound = 3, UpperBound = 4 },
  ...
};

using var inputDocument = PdfReader.Open(inputFile, PdfDocumentOpenMode.Import);

foreach(var range in pageRanges.Select((pageRange, index) => new { Pages = pageRange, Index = index })) {
  var outputDocument = new PdfDocument();
  outputDocument.Options.UseFlateDecoderForJpegImages = PdfUseFlateDecoderForJpegImages.Automatic;
  outputDocument.Options.FlateEncodeMode = PdfFlateEncodeMode.BestCompression;
  outputDocument.Options.EnableCcittCompressionForBilevelImages = true;
  outputDocument.Options.CompressContentStreams = true;
  outputDocument.Options.NoCompression = false;
  outputDocument.Version = inputDocument.Version;
  var pageInfo = $"Pages {range.Pages.LowerBound}-{range.Pages.UpperBound}";
  outputDocument.Info.Title = $"{pageInfo} of {inputDocument.Info.Title}";
  outputDocument.Info.Creator = inputDocument.Info.Creator;
  for (var pageNumber = range.Pages.LowerBound; pageNumber <= range.Pages.UpperBound; pageNumber++)
  {
      outputDocument.AddPage(inputDocument.Pages[pageNumber - 1]);
  }
  
  var outputPath = Path.Combine(targetDirectory, $"{filenameBase}_part_{range.Index + 1}.pdf");
  outputDocument.Save(outputPath);
}