Sometimes when I try to combine Pdf files, a page would be duplicated where the next page
is supposed to be.

Here is the code I use to combine the pdf files.

    pdfFiles = []
    for filename in os.listdir(self.source):
        if filename.endswith('.pdf'):
            pdfFiles.append(os.path.join(self.source,filename))


    pdfFiles.sort(key = str.lower)

    pdfWriter = PyPDF2.PdfWriter()
    for filename in pdfFiles:
        #print(filename)
        pdfFileObj = open(filename,'rb')
        pdfReader = PyPDF2.PdfReader(pdfFileObj)
        #for pageNum in range(0, pdfReader.numPages):
        for pageNum in range(len(pdfReader.pages)):
            #pageObj = pdfReader.getPage(pageNum)
            pageObj = pdfReader.pages[pageNum]
            pdfWriter.add_page(pageObj)
            pdfFileObj.close()

    fileout = CombineName
    #print(fileout)
    pdfOutput = open(self.destination + fileout,'wb')
    pdfWriter.write(pdfOutput)
    pdfOutput.close()

The number pages are correct but some pages are duplicated and the other page is missing.