Any food safety risk eating cold refrigerated pizza?
Not really. Assuming that the pizza has been cooled down and stored properly (for which see here and here) then it most likely won't have developed a potentially harmful microbial load. Additionally, while reheating might kill off most of the microbes in the food (assuming that you reached and maintained a temperature sufficient for pasteurization, which isn't a given) that still wouldn't render the food completely safe, because even dead microbes can leave behind toxic substances that can make you sick.

Be warned that just because you haven't gotten sick in past, that doesn't mean your method is completely safe. Food safety is a science of relative risk, not absolute certainty, and getting lucky 99 times in a row doesn't mean that the 100th time won't be a miserable one.

But, bottom line: put your pizza into the fridge when you're done eating it, and don't keep it for more than a couple days or if it shows obvious signs of spoilage, and most likely you'll be okay. Reheating is not essential.