Puffed pack means there is a gas inside the packet and this gas is produced due to fermentation caused by a microbe. Presence of microbe makes the drink unhealthy and bad that is why it is written on the pack " If the pack is puffed don't buy it".
Anymicrobethat lives in the drink makes a gas after fermentation In case of Yeast, Carbon dioxide is the gas.So if you see a puffed pack, it means that there has been achemical reactionthat made the drink go bad.
Maitreya Pathade answered this
Because it may mean that fermentation of the juice has started and that just means its close to expiring, I wouldn't recomend drinking it, just check the dates!!