Consistency can be solved with hashing and cryptographic signatures. The user story here is that PDFs get sent around as forms to be filled out, and that poses

Consistency can be solved with hashing and cryptographic signatures. The user story here is that PDFs get sent around as forms to be filled out, and that poses a problem for non Mac users or users without sufficient technical skill. And since you reference mp3 and jpg, you surely know that both formats can be modified in ways that many people will not recognize as modifications. It just pushes the skill level up a bit. But there's always a technically capable person available for hire to modify one of the "permanent" formats you mention.