Polyacrylonitrile (PAN)-based carbon fiber papers (CFPs) intended for use as gas diffusion layers in proton exchange membrane fuel cells were prepared, and the morphological, physical, mechanical, and electrical properties of the CFPs were investigated. Ultrafine PAN fiber additives in the CFPs form crosslinks between the carbon fibers, increasing the bulk densities of the CFPs and improving their mechanical and electrical properties. The binding force of the CFPs increased with the addition of fibrid additives, but the overall tensile strength decreased below the critical value. The surface resistivity of the CFPs increased with increasing fibrid content though the surface density was held constant.