Biff stole while growing up and he steals as an adult. He takes what he wants without hesitation. He blames his father for this habit because Willy did not punish him for stealing when he was young and instill in him a sense of integrity.
Biff had gone to see Bill Oliver to pitch a deal and borrow money. Oliver keeps him waiting for hours, which Biff surely resented, and then dismisses him quickly. Biff, left alone, goes into Oliver's office, where he certainly does not belong, and takes the expensive pen. In one regard, sneaking into Oliver's office and stealing his pen is a petty, angry reaction to having been kept waiting and then rejected by Oliver. Biff tells his brother Happy, "[H]e gave me one look and--I realized what a ridiculous lie my whole life has been!" Oliver's success reminds Biff of his own life's failure. Taking the pen was also an impulse, a crime of opportunity in which Biff's basic character asserts itself.
Always one to avoid responsibility, Biff enlists Happy's aid in lying to Willy about stealing the pen. Without any hesitation, Happy covers for Biff with their father, making up a story to make Biff seem innocent of any misdeed.