Save a file from PDF Writer

Question

I have created a variable through a PdfWriter and add to it pages from other PDF document. How can I save variable from PdfWriter as PDF in GoogleCollab?

pdf_writer_new = PdfWriter()

for page in range(totalPages):
    if num == 7:
      if page in pages_7:
        pdf_writer_new.add_page(pdf.pages[page])
    elif num == 8:
      if page in pages_8:
        pdf_writer_new.add_page(pdf.pages[page])
    elif num == 9:
      if page in pages_9:
        pdf_writer_new.add_page(pdf.pages[page]) 

I have tried several methods, but they didn't work. In each case GoogleCollab return that there is no such property in PdfWriter.