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Physically Unclonable Functions (PUFs) are emerging hardware security primitives that leverage random variations during chip manufacturing process to generate unique secrets. The security level of generated PUF secrets is mainly determined by its unpredictability feature which is typically evaluated using the metric of entropy bits. In this paper, we propose a novel Pairwise Distinct-Modulus (PDM) technique that significantly improves the upper bound of PUF entropy bits from the scale of log2(N!) up to O(N^2). The PDM technique boosts entropy by eliminating the correlation within PUF response bits caused by element reuse in conventional pairwise comparison. We also propose a reliability-enhancing scheme to compensate the impact on reducing reliability by saving a significant portion of potential reliable response bits. Experimental results based on a published large-scale RO PUF frequency dataset validated that the proposed technique significantly boosts PUF entropy bits from the scale of O(N∙log2(N)) up to approach the new upper bound of O(N^2) with a comparable reliability, and the reliability-enhancing technique saves 4x more on the percentage of reliable response bits.