PDFsharp & MigraDoc Foundation

    public Stream[] SplitDocument(Stream fileStream, int[] splitOnPageNumbers)
    {
      PdfDocument inputDocument = PdfReader.Open(fileStream, PdfDocumentOpenMode.Import);

      List<Stream> result = new List<Stream>();
      PdfDocument outputDocument = null;

      for (int page = 0; page < inputDocument.PageCount; page++)
      {
        // when there is no active document; create one
        if (outputDocument == null)
        {
          outputDocument = new PdfDocument();
          outputDocument.Version = inputDocument.Version;
          outputDocument.Info.Title = inputDocument.Info.Title;
          outputDocument.Info.Creator = inputDocument.Info.Creator;
        }

        // add the page and save to stream
        outputDocument.AddPage(inputDocument.Pages[page]);

        // when this page was indicated as a split number; save the document
        // also save by default when it's the last page
        // add 1 to the page number because that's zero based, and the splitOnPageNumbers var is one based
        if (splitOnPageNumbers.Contains(page + 1) || page + 1 == inputDocument.PageCount)
        {
          Stream outputStream = new MemoryStream();
          outputDocument.Save(outputStream, false);       // don't close stream, it will be unusable
          result.Add(outputStream);

          outputDocument = null;                          // reset to null to spawn a new document
        }
      }

      return result.ToArray();
    }