Technically, the %PDF doesn't have to occur at byte 0. But, like many others inferred, that pushes onward toward real world structure (vs. an academically correct but useless PDF).

If you want a fully covered example it'll need a trailer, xref and at least one obj reference. ...and there are TWO flavors of those (linearized and not). ...and flate coded and not.

So, for a test harness you'd actually want a collection of small files.