This copy is for your personal, non-commercial use only. To order presentation-ready copies for distribution to your colleagues, clients or customers visit http://www.djreprints.com. At age 53, the activist investor Bill Ackman is having the best year of his career. And for those who want to invest alongside him, there is now an opportunity to get in at a big discount. Ackman’s closed-end fund, Pershing Square Holdings (ticker PSH. Netherlands), closed Tuesday at $17.90 a share, a 28% discount to its net asset value of $24.92 on June 30.