public static void SplitPDFDocuments(string sourceDocumentFilename, params string[] resultDocumentsFilenames)
{
    if (resultDocumentsFilenames.Length == 0)
        return;
    // open source document
    Vintasoft.Imaging.Pdf.PdfDocument sourceDocument = 
        new Vintasoft.Imaging.Pdf.PdfDocument(sourceDocumentFilename);
    // create result documents array
    int resultDocumentsLength = System.Math.Min(resultDocumentsFilenames.Length, sourceDocument.Pages.Count);
    Vintasoft.Imaging.Pdf.PdfDocument[] resultDocuments = 
        new Vintasoft.Imaging.Pdf.PdfDocument[resultDocumentsLength];
    for (int i = 0; i < resultDocuments.Length; i++)
        // create result document v1.4
        resultDocuments[i] = new Vintasoft.Imaging.Pdf.PdfDocument(
            resultDocumentsFilenames[i], System.IO.FileMode.Create, Vintasoft.Imaging.Pdf.PdfFormat.Pdf_14);

    // split source document
    int currenPageIndex = 0;
    int totalPages = sourceDocument.Pages.Count;
    while (currenPageIndex < totalPages)
    {
        for (int i = 0; i < resultDocuments.Length; i++)
        {
            resultDocuments[i].Pages.Add(sourceDocument.Pages[currenPageIndex]);
            currenPageIndex++;
            if (currenPageIndex >= totalPages)
                break;
        }
    }

    for (int i = 0; i < resultDocuments.Length; i++)
    {
        // save changes
        resultDocuments[i].SaveChanges();
        // close result document
        resultDocuments[i].Dispose();
    }
    // close source document
    sourceDocument.Dispose();
}

Public Shared Sub SplitPDFDocuments(sourceDocumentFilename As String, ParamArray resultDocumentsFilenames As String())
    If resultDocumentsFilenames.Length = 0 Then
        Return
    End If
    ' open source document
    Dim sourceDocument As New Vintasoft.Imaging.Pdf.PdfDocument(sourceDocumentFilename)
    ' create result documents array
    Dim resultDocumentsLength As Integer = System.Math.Min(resultDocumentsFilenames.Length, sourceDocument.Pages.Count)
    Dim resultDocuments As Vintasoft.Imaging.Pdf.PdfDocument() = New Vintasoft.Imaging.Pdf.PdfDocument(resultDocumentsLength - 1) {}
    For i As Integer = 0 To resultDocuments.Length - 1
        ' create result document v1.4
        resultDocuments(i) = New Vintasoft.Imaging.Pdf.PdfDocument(resultDocumentsFilenames(i), System.IO.FileMode.Create, Vintasoft.Imaging.Pdf.PdfFormat.Pdf_14)
    Next

    ' split source document
    Dim currenPageIndex As Integer = 0
    Dim totalPages As Integer = sourceDocument.Pages.Count
    While currenPageIndex < totalPages
        For i As Integer = 0 To resultDocuments.Length - 1
            resultDocuments(i).Pages.Add(sourceDocument.Pages(currenPageIndex))
            currenPageIndex += 1
            If currenPageIndex >= totalPages Then
                Exit For
            End If
        Next
    End While

    For i As Integer = 0 To resultDocuments.Length - 1
        ' save changes
        resultDocuments(i).SaveChanges()
        ' close result document
        resultDocuments(i).Dispose()
    Next
    ' close source document
    sourceDocument.Dispose()
End Sub