The Legacy of Pangu

Legend has it that Pangu exhausted all his magic power creating heaven and earth. Before he passed away, he hid a divine cultivation manual somewhere within the Three Realms. The Three Pure Ones searched for billions of years without finding it, and had no choice but to forge another path and cultivate divine bodies. The Three Pure Ones wielded such immense power, so why did they fail to find the manual even after searching for billions of years? Was it all just a legend, and did Pangu leave no manual behind before he passed away?