Plays a key role in early steps of protein N-linked glycosylation by being required for the conversion of polyprenol into dolichol. Dolichols are required for the synthesis of dolichol-linked monosaccharides and the oligosaccharide precursor used for N-glycosylation. Acts as a polyprenol reductase that promotes the reduction of the alpha-isoprene unit of polyprenols into dolichols in a NADP-dependent mechanism (By similarity). di-trans,poly-cis-dolichol + NADP(+) = di-trans,cis-polyprenol + H(+) + NADPH Protein modification; protein glycosylation. Belongs to the steroid 5-alpha reductase family. Polyprenol reductase subfamily.