My grand father had adenocarcinoma. what causes it? how do you get it?

Multiple causes: Adenocarcinoma is a cancer that arises from glandular elements in various organs. It is a general term for a number of cancers, including those arising from breast, lung, stomach, pancreas, and colon. These different diseases have different causes, and for many cases the cause is unknown. Your question did not give enough information to allow a more detailed response.