Code:
Private Sub MergeMultiplePDFIntoSinglePDF(ByVal outputFilePath As String, ByVal pdfFiles As String())
        Dim document As PdfDocument = New PdfDocument()
        For Each pdfFile As String In pdfFiles
            Dim inputPDFDocument As PdfDocument = PdfReader.Open(pdfFile, PdfDocumentOpenMode.Import)
            document.Version = inputPDFDocument.Version
            For Each page As PdfPage In inputPDFDocument.Pages
                document.AddPage(page)
            Next
            'System.IO.File.Delete(pdfFile) '被防毒軟體擋下來,以後再改
        Next
        document.Options.CompressContentStreams = True
        document.Options.NoCompression = False
        document.Save(outputFilePath)
    End Sub