The páo reading preserves the original sense of the character: a cooking/roasting method in which food (or, later, herbs) is wrapped and dry-heated. From Han times this method became standard in Chinese medicine — soaking, drying, frying, calcining herbs to change their properties. Today
炮制 has split into two readings of its own: the literal "to roast-process herbs" and the figurative "to concoct / fabricate (fake news, a forged document, a hatchet job)" — both pronounced páozhì.