My thought is that all pepperoni is not created equal.
I use Margherita's traditional stick pepperoni, it will keep in the refrigerator for a long, long time.
Margherita® | There's Italian, And Then There's Margherita
I would be concerned about storing some of the pepperoni products that come sliced paper thin in pillow packs or some of the stick pepperoni that seems to have a high moisture content.
Another thought, if the leftover pepperoni goes bad in the freezer, try finding new uses for it.
I chop it up and add it to scrambled eggs or add part of a stick to Sunday Gravy.