SEATTLE — Paul G. Allen, who co-founded Microsoft with his childhood friend Bill Gates before becoming a billionaire philanthropist who invested in conservation, space travel and professional sports, died Monday. He was 65. He died in Seattle from complications of non-Hodgkin’s lymphoma, his company Vulcan Inc. announced. Microsoft CEO Satya Nadella called Allen’s contributions to the company, community and industry “indispensable.” “As co-founder of Microsoft, in his own quiet and persistent way, he created magical products, experiences and institutions, and in doing so, he changed the world,” Nadella wrote on Twitter. Two weeks ago, Allen announced that the non-Hodgkin’s lymphoma that he was treated for in 2009 had returned and he planned to fight it aggressively. “My brother was a remarkable individual on every level,” Allen’s sister Jody Allen said in a statement.