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)]
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