Acrylic ships with a brown adhesive backing to protect it from scratches. We remove this backing on the side we etch to ensure that the etch is nice and detailed. We will typically leave the paper on the other side to keep things while they are stored in bins and ultimately shipped to you.
This paper usually just peels right off. If you are finding it hard to remove you can soak the piece in soapy water to soften the paper.