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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)]