Document classification is the detection specific content of interest in text documents. In contrast to the data-driven machine learning classifiers, knowledge-based classifiers can be constructed based on domain specific knowledge, which usually takes the form of a collection of subject related keywords. While typical knowledge-based classifiers compute a prediction score based on the keyword abundance, it generally suffers from noisy detections due to the lack of guiding principle in gauging the keyword matches. In this paper, we propose a novel knowledge-based model equipped with Shannon Entropy, which measures the richness of information and favors uniform and diverse keyword matches. Without invoking any positive sample, such method provides a simple and explainable solution for document classification. We show that the Shannon Entropy significantly improves the recall at fixed level of false positive rate. Also, we show that the model is more robust against change of data distribution at inference while compared with traditional machine learning, particularly when the positive training samples are very limited.