This work reports on the preparation of pinewood residues/recycled high-density polyethylene (HDPE) composites to evaluate their performance under flexion, extraction of nails and screws, and moisture absorption (MA) to assess their potential to replace medium-density fiberboards (MDFs). The effect of filler particle size (PS) was evaluated, and scanning electron microscopy (SEM) was conducted to elucidate the state of the interphase. The effect of UV-light accelerated weathering (AW) on samples with and without a UV stabilizer (UVS) was assessed. A dynamic mechanical analysis (DMA) was also conducted. The composites had better flexural performance, MA, and screw extraction resistance than the MDFs. However, AW affected the composites, mostly affecting those without UVS. Scanning electron microscopy showed the appearance of cracks on the surfaces with less UVS. The DMA results suggested that the composites with the largest PS showed a better resistance to creep.