Technically, the island of Taiwan is not expressly a province of China. It is a de facto province of the Republic of China, outlined by intent of the agreement by the Allies in the Cairo Declaration. But the Cairo Declaration is unsigned, and is a declaration of intent. As such, it doesn't hold the force of a treaty, and only the allowance by the U.S. of the Republic of China taking possession and administrative responsibility for the island following the end of the war that effectively transfers possession from the U.S. to the Republic of China.
I'll note here that it is probably also important to note that Japan surrendered to the United States, and thus the unconditional surrender of Japan limited Japan to its main islands and stripped it of all other possessions. By historical standards, to the victor go the spoils, and by default the United States is the true title holder to the island of Taiwan. De facto ownership was transferred to the Republic of China, and the Republic of China remains in de facto possession to this day.
As the history texts show, the Communists mounted a rebellion and fought a civil war against the Republic of China. This civil war remains unresolved as both the rebellious government, the PRC, and the original government, ROC, both exist today. The ROC retains defacto possession of Taiwan, which it held by agreement with the United States following WWII.
So, either the island belongs to the Republic of China, or it belongs to the United States. Either way, the People's Republic has no claim.
Actually, the PRC uses the Cairo declaration as one of the historical evidence for making a claim on Taiwan and the Taiwan Independencement movement is the one trying to minimize it.
Also, keep in mind, the original polital party in power during the Cairo Declaration, the Kuomintang, continues to this day, to work towards re-unification with China.