docs: def _get_pdf_filenames_at(source_directory):
      """Find all PDF files in the specified directory.
  
      Args:
          source_directory (str): The source directory.
  
      Returns:
          list(str): Filepaths to all PDF files in the specified directory.
  
      Raises:
          ValueError
      """
      if not os.path.isdir(source_directory):
          raise ValueError("%s is not a directory!" % source_directory)
      return [os.path.join(source_directory, filename)
              for filename in os.listdir(source_directory)
              if filename.endswith(PDF_EXTENSION)]