A mono or multi-layer biaxially oriented polyester film is described. Also described, is a process for producing the mono or multi-layer biaxially oriented polyester film. Uses of a surface-treated filler material product as a voiding agent in the mono or multi-layer biaxially oriented polyester film, an article that includes the mono or multi-layer biaxially oriented polyester film as well as the use of the mono or multi-layer biaxially oriented polyester film in packaging products are also described. The packaging products can include: paper and glass coverings, insulating materials, solar, marine and aviation applications, science, electronic and acoustic applications, wires, cables, radio frequency identifications, flexible circuits, graphic arts, filter products, cosmetic products, household products, imaging and recording media, or industrial products.