Title: A Critical Analysis of Out-of-Distribution Detection for Document Understanding. Abstract: Large-scale pretraining is widely used in recent document understanding models. During deployment, one may expect that large-scale pretrained models should trigger a conservative fallback policy when encountering out-of-distribution (OOD) samples, which suggests the importance of OOD detection. However, most existing OOD detection methods focus on single-modal inputs such as images or texts. While documents are multi-modal in nature, it is underexplored if and how multi-modal information in documents can be exploited for OOD detection. In this work, we first provide a systematic and in-depth analysis on OOD detection for document understanding models. We study the effects of model modality, pretraining, and finetuning across various types of OOD inputs. In particular, we find that spatial information is critical for document OOD detection. To better exploit spatial information, we propose a simple yet effective special-aware adapter, which serves as an add-on module to adapt transformer-based language models to document domain. Extensive experiments show that our method consistently improves ID accuracy and OOD detection performance compared to baselines. We hope our findings can help inspire future works on understanding OOD robustness for documents.