Is sticky pepperoni a sign of spoilage?
Pepperoni has quite a bit of fat in it, and it's going to break down over time as it ages in the fridge (just like the meat that went into it). That can result in a sort of tacky feeling to it. You get this in things like beef summer sausage, too. It's always rather dry by the time it spoils, which takes quite a while. 

Sticking an uncovered sausage in the back of the ref for a day or so is something many folks do to bring out the spices it contains a bit more prior to cooking. It also tends to come out a little tacky, depending on fat and preservative content. I do this with store-bought Italian sausage to get the fennel and anise a bit more pronounced.

If it doesn't smell acrid or rancid, and feels like pepperoni despite being a little tacky, it's probably fine. When preserved meats go bad a few parts of it start to have an acrid sort of odor, quickly followed by the rest. It's harder to spot initially in chorizo and similar, but bad pepperoni really gives itself away pretty quickly. 

Also, taste a small piece. If it's rather sour, you probably don't want to use it.

Source: Me always being given the job of cleaning out and rotating stuff in the walk-in in at a place I used to work that made a lot of pizza.