You will hear this term everywhere when you go to China.
PK in China means fighting one-on-one or comparison. The Acer poster above is saying "If you don't like it then PK" roughly meaning if you don't like the price then let's compare (with your laptop). I don't think it means fighting one-on-one here.
The use of "PK" became popular when it was used in the Super Girl Contest where it meant one contestant was challenged head-to-head by another contestant, for example, Li Yuchun PK Zhang Liangying. When I first heard it from my Chinese colleagues, I thought they were talking about stuff related to boxing (I mixed it up with KO). Later I found out originally PK meant "player killing" in on-line gaming or "penalty kick" in soccer.
According to one report, "PK" is now possibly the No.5 most well known English word after "Yes", "No", "Bye-bye" and "OK".
PK is especially a powerful term to use in marketing. It is widely used in product comparison, eg, Fuji PK Canon. I found this "Google PK Platform" website where you can look up different "PK value". Although I am not quite sure if it is owned by Google since the font looks a little odd:
Please note, that "PK" can be interpreted as "Puk Kai" in Hong Kong, and it means "go to hell" or "asshole" (it is a versatile term that can be used as a noun, verb and an adjective). So be careful with your PK use.