It sounds like what they want is a bit like what you get with a word processor provided that everyone is using the same version of the same program on identical

It sounds like what they want is a bit like what you get with a word processor provided that everyone is using the same version of the same program on identical systems, so you don't have the current situation of the layout getting completely broken because different people have different fonts, different paper sizes, and so on. In which case it's an interesting idea, but they shouldn't call it "PDF". Although it's an interesting idea, I suspect it will never work in practice because word processing is just too complex. There are just too many complex features that people expect to have available. Different implementations will never be sufficiently compatible. Perhaps the solution is to bundle your document with a WebAssembly binary of a particular version of LibreOffice? OK, maybe you could separate the rendering functionality from the UI stuff, but it's hard to see how in practice you could get documents to be editable and rendered in the same way everywhere except by having everyone run the same binary to do the rendering, and there will inevitable be a hundred versions of that binary in use as new features get added.