Cu/PET film composites were prepared by electroless copper plating method. In order to improve adhesion between electroless Cu plated layer and polyester (PET) film, the effect of pretreatment conditions such as etching method and mixed catalyst composition, and accelerator was investigated. Compared to NaOH etching medium, PET film was more finely etched by HCl solution, resulting in an improvement in adhesion between Cu layer and PET film. However, there were no significant differences in electromagnetic interference shielding effectiveness as a function of etching medium. The surface morphology of Cu plated PET film revealed that Pd/Sn colloidal particles became more evenly distributed in the smaller size by increasing the molar ratio of PdCl$_2$ : SnCl$_2$ from 1 : 4 to 1 : 16. With increasing the molar ratio of mixed catalyst, the adhesion and the shielding effectiveness of Cu plated PET film were increased. Furthermore, HCl was turned out to be a better accelerator than NaOH in order to enhance the activity of the mixed PdCl$_2$/SnCl$_2$ catalyst, which facilitated the formation of more uniform copper deposit on the PET film.