Duplex cover is two sheets of quality text laminated together, usually two colors. It can be quite good for artwork and book arts because of the stiffness and the design options offered by the two colors. Sized means there is some gelatin or polymer applied to the paper surface or into the pulp to make the paper stronger and less likely to soak up moisture. Pulp is the material paper is made from. Wood chips are cooked in acid to break up the fibers.
Residual acid can react with things in the air and make paper weak over time.
Plenty of water is needed to clean the pulp enough to make good paper. Ph refers to the degree of acidity or alkalinity in paper pulp. Most machine made paper is acidic, and mills now put things like calcium carbonate in to counteract this and to allow a Ph neutral label. This treatment affects the performance for surface design, and I have found very little of this new Ph neutral paper to be useful. The pattern rinses off or the applied paints soak right through the sheets.