Can pinhole glasses help improve eyesight or reduce eye strain?

Blurry vision. Pinhole glasses, like squinting, help to refine your focus generally with distance viewing. The pinholes limit the amount of light entering your eyes and therefore are not practical way of helping your vision. The ophthalmologist uses pinhole acuity to help determine whether a refractive error exists. The pinhole glasses are harmless. Better approach is just getting a refraction for glasses.
Yes. Pinhole can help sharpen vision, but at the expense of decreasing your field of vision (see straight ahead only with no peripheral). Eye strain can be reduced by having the proper refractive correction.