def _get_pdf_filenames_at(source_directory):
if not os.path.isdir(source_directory):<EOL><INDENT>raise ValueError("<STR_LIT>" % source_directory)<EOL><DEDENT>return [os.path.join(source_directory, filename)<EOL>for filename in os.listdir(source_directory)<EOL>if filename.endswith(PDF_EXTENSION)]<EOL>
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