code: def get_concatenated_pdf_from_disk(filenames: Iterable[str],
                                     start_recto: bool = True) -> bytes:
      
      # http://stackoverflow.com/questions/17104926/pypdf-merging-multiple-pdf-files-into-one-pdf  # noqa
      # https://en.wikipedia.org/wiki/Recto_and_verso
      if start_recto:
          writer = PdfFileWriter()
          for filename in filenames:
              if filename:
                  if writer.getNumPages() % 2 != 0:
                      writer.addBlankPage()
                  writer.appendPagesFromReader(
                      PdfFileReader(open(filename, 'rb')))
          return pdf_from_writer(writer)
      else:
          merger = PdfFileMerger()
          for filename in filenames:
              if filename:
                  merger.append(open(filename, 'rb'))
          return pdf_from_writer(merger)
docs: Concatenates PDFs from disk and returns them as an in-memory binary PDF.
  
      Args:
          filenames: iterable of filenames of PDFs to concatenate
          start_recto: start a new right-hand page for each new PDF?
  
      Returns:
          concatenated PDF, as ``bytes``