I've never heard of Pepsi Max, but I'll assume it's similar to Pepsi
One (their diet cola in the U.S.). According to the Pepsi web site:
http://www.pepsi.com/pepsi_brands/product_info/
pepsione/index.php
Pepsi ONE (8 fl. oz)
Contains: Carbonated water, caramel color, natural and artificial flavors, phosphoric acid, aspartame, citric acid, sodium benzoate (preserves freshness), caffeine and acesulfame potassium
Calories 1
Total Fats (g) 0
Sodium (mg) 30
Potassium (mg) 20
Total Carbohydrates (g) 0
Sugars (g) 0
Protein (g) 0
Caffeine (mg) 37
An 8 oz. serving in the U.S. is about the same as 236ml -- divide 5000ml (or 5 liters) by this number and you get about 21.2 eight oz. servings.
That would yield 21.2 calories, with little to no protein, fats, or carbs. So no, one would not get "fat" from drinking 5 liters per day, assuming they didn't eat 21 burgers or bags of potato chips to go with it.
Why anyone would *want* to drink that much, however, I couldn't say...