KAP: MLLM-assisted OCR Text Enhancement for Hybrid Retrieval in Chinese Non-Narrative Documents

Abstract: Hybrid Retrieval systems, combining Sparse and Dense Retrieval methods, struggle with Traditional Chinese non-narrative documents due to their complex formatting, rich vocabulary, and the insufficient understanding of Chinese synonyms by common embedding models. Previous approaches inadequately address the dual needs of these systems, focusing mainly on general text quality improvement rather than optimizing for retrieval. We propose Knowledge-Aware Preprocessing (KAP), a novel framework that transforms noisy OCR outputs into retrieval-optimized text. KAP adopts a two-stage approach: it first extracts text using OCR, then employs Multimodal LLMs to refine the output by integrating visual information from the original documents. This design reduces OCR noise, reconstructs structural elements, and formats the text to satisfy the distinct requirements of sparse and dense retrieval. Empirical results demonstrate that KAP consistently and significantly outperforms conventional preprocessing approaches. Our code is available at https://github.com/JustinHsu1019/KAP.