Dear I used to have an old version of my apps
it was working perfectly so I didn't touch the formula or anything ,,, it expand the gallery perfectly without any errors put so on so far ,,, I think when I update the apps for different modifications not related to this pdf function, the capability is gone ,,,, the last publishing was one year from now ,,, when I update this august the capability gone .... something related to platform update happens by MS them self