A mono or multi-layer polyester film is described. Also described, is a process for producing the mono or multi-layer polyester film, the use of a surface-treated filler material product as a voiding agent in the mono or multi-layer polyester film, and an article including the mono or multi-layer polyester film. Further, use of the mono or multi-layer polyester film is described in packaging products, insulating materials, solar, marine or aviation applications, science, electronic or acoustic applications, wires, cables, radio frequency identifications, flexible circuits, graphic arts, stone paper, holograms, filter products, cosmetic products, household products, imaging, recording media, or industrial products.