My two cents : PRDs are based on the inside out philosophy. I as the product manager, through various modes of validation, come up with the requirements that I feel will add value to customers. When that is put in detail in a spec, it turns out to be a monolithic script drawing little to no attention. Most of the engineers miss out a lot of stuff in the spec.
The other way that I see working very well for nimble and agile organizations is the format of User Stories. Here, I do not write from a product manager’s perspective, but rather from a customer’s perspective (As a user, I would like to search the top rated items … etc. etc. ). THis becomes an outside in philosophy. Easier and more engaging to read and follow for all , especially the engineering team members. Caveat : The org structure should also be matured to adapt this and implement it in the true sense of Scrum way of product development.