Bill Ackman's Pershing Square Capital Management gained 7.9 percent net of fees for the month of October in its largest fund, dramatically reversing the year-to-date returns of the outspoken activist investor.

Pershing Square International is now up 8.1 percent for the year, according to an investor performance update obtained by CNBC.com. The same fund had been up just 0.2 percent through September.