Understanding the multimodal documents is essential for accurately extracting relevant evidence and using it for reasoning. Existing document understanding models struggle to focus on key information and tend to generate answers straightforwardly, ignoring evidence from source documents and lacking interpretability. In this work, we improve the visual encoder to focus on key information relevant to the question and address the shortcomings of existing document visual question-answering datasets to provide the model with the ability to answer questions step-wise, dubbed DocAssistant. Specifically, for the visual side, we propose an effective vision-language adaptation that fuses text into visual encoders without compromising the performance of the original model. For the language side, we use Multimodal Large Language Models (MLLMs) as data generators and checkers to produce high-quality step-wise question-and-answer pairs for document images. We then use the generated high-quality data to train our enhanced model, specifically designed to solve complex questions that require reasoning or multi-hop question answering. The experimental results demonstrate the effectiveness of the model.