are you sure it's a upvc door? not commonly used internally. i suspect that it's an engineered door (probably hollow) with a textured board. they can be sanded and painted.
if it IS upvc then it should not be too much of a problem to gently scrape it off once the gloss is fully dry (assuming it was a solvent based paint) which might take several weeks. in fact it will probably start peeling of it's own accord. avoid paint stripping chemicals as they will damage the pvc.