Strength of board is directly related to density, but WPC/PVC boards carry very low density, hence how do you compare these boards with Plywood?

For this we need to understand what is strength and what is the basic reason that imparts strength to a material. Strength measures the resistance of a material to failure given by the applied stress.The strength of any material  is directly related to density but it depends on the direction of the grains and the cellular microstructure. Every plywood may not be stronger than WPC/ PVC  and vice versa. The whole strength depends on its cellular structure. In plywood , it is natural to have that but in WPC , fine celled foam structure is  achieved through technology.Also WPC is not just PVC , it is wood polymer composites. The thermoplastic ( polymer )provides the required flexibility and rice husk/ wheat husk  imparts the required strength.Most polymers have densities slightly greater than 1 and  most woods slightly less than 1 High strength at low weight is so often important that a property called specific strength is defined as strength/density. And WPC boards have low density because of thermoplastic and better strength due to husk. There are applications which require low weight and better strength , for these applications ,composites provide a means of achieving high strength at low weight because they exploit very strong fibres in light matrices