public static void CombinePDFDocuments(string[] sourceDocuments, string resultDocument)
{
    // create new PDF document version 1.6 with compressed cross-reference table
    Vintasoft.Imaging.Pdf.PdfDocument document = new Vintasoft.Imaging.Pdf.PdfDocument(
        resultDocument,
        System.IO.FileMode.Create,
        new Vintasoft.Imaging.Pdf.PdfFormat("1.6", true, true));

    // for each source document
    for (int i = 0; i < sourceDocuments.Length; i++)
    {
        // open the source document in read-only mode
        Vintasoft.Imaging.Pdf.PdfDocument sourceDocument = 
            new Vintasoft.Imaging.Pdf.PdfDocument(sourceDocuments[i], true);
        // add pages from source document to result document
        document.Pages.AddRange(sourceDocument.Pages.ToArray());
        // close the source document
        sourceDocument.Dispose();
        // save changes to a file
        document.SaveChanges();
    }

    // close result document
    document.Dispose();
}

Public Shared Sub CombinePDFDocuments(sourceDocuments As String(), resultDocument As String)
    ' create new PDF document version 1.6 with compressed cross-reference table
    Dim document As New Vintasoft.Imaging.Pdf.PdfDocument(resultDocument, System.IO.FileMode.Create, New Vintasoft.Imaging.Pdf.PdfFormat("1.6", True, True))

    ' for each source document
    For i As Integer = 0 To sourceDocuments.Length - 1
        ' open the source document in read-only mode
        Dim sourceDocument As New Vintasoft.Imaging.Pdf.PdfDocument(sourceDocuments(i), True)
        ' add pages from source document to result document
        document.Pages.AddRange(sourceDocument.Pages.ToArray())
        ' close the source document
        sourceDocument.Dispose()
        ' save changes to a file
        document.SaveChanges()
    Next

    ' close result document
    document.Dispose()
End Sub