The Cantonese for Pepsi is baak sih, which sort of sounds like Pepsi (the Mandarin pronuncation bǎi shì is further off). Technically, it's short for 百事可樂, Pepsi-Cola (baak sih hó lohk), but as in English, people usually don't bother saying the full name (and which is why in Hong Kong, I would order Pepsi more often than Coke because I found the Cantonese for Pepsi easier to say than the Cantonese for cola).