Function process_batch 

pub fn process_batch<P, F, T>(
    paths: &[P],
    config: BatchConfig,
    processor: F,
) -> BatchResult<T>
where
    P: AsRef<Path>,
    F: Fn(&PdfDocument) -> Result<T> + Send + Sync + 'static,
    T: Send + 'static,

Process a list of PDF paths with a fixed-size worker pool.