I guess I'd call it mostly fixed: most TeX engines only produce a single page size at a time, but you generally only need to modify a single variable to change the page size and, after recompiling the document should be perfectly reflowed into the new page size. This is not helpful at all for someone who wants to be able to freely rotate their phone, but it's still better than InDesign or Word where changing the page size will probably break the layout.

There is actually a TeX extension that supports viewers arbitrarily changing the page size of documents [0], but it's fairly new, and as far as I'm aware it has essentially zero adoption.

[0]: https://ctan.org/pkg/hitex