The series is tied 3-3 going into Game 7 on Wednesday night in Boston.
Pacioretty, who had only one assist in the first five games, scored and set up Thomas Vanek's goal in the second period. Lars Eller scored in the first for Montreal and Vanek added his second of the game into an empty net with 3:56 left.
Rookie Nathan Beaulieu, the Canadiens' 2011 first-round draft pick who was given a surprise start by coach Michel Therrien, picked up an assist in his first NHL playoff game and was plus-2.
The Canadiens repeated the scenario of their 2011 first-round series against the Bruins, winning at home to force a Game 7. That year, they lost Game 7 in overtime. The Bruins went on to win the Stanley Cup.
Montreal got out to a strong start after a rousing pregame show and the teams played at a ferocious pace through the first two periods.
Despite having the best of the play in the opening 20 minutes, the Canadiens needed an unusual play for the only goal.
Kevan Miller lost the puck off his stick behind the Boston net and then inadvertently tripped goalie Tuukka Rask as he tried to smother it, leaving Eller free to score unassisted 2:11 into the game.
DUCKS 4, KINGS 3
ANAHEIM, Calif. (AP) — Devante Smith-Pelly scored two goals 1:23 apart in the second period, and Anaheim moved to the brink of the Western Conference finals with a victory over Los Angeles in Game 5.
John Gibson made 39 saves in his second career playoff start as the Ducks took a 3-2 lead in the second-round series with their third straight victory.
Nick Bonino and Jakob Silfverberg also scored for the Ducks, who jumped to a three-goal lead in the second period and hung on behind their 20-year-old rookie goalie, earning the first home victory by either team in the series.
Marian Gaborik scored two goals for the Kings, who will face their fifth elimination game of the post-season.
Game 6 is Wednesday at Staples Center.
Gaborik scored his NHL-best seventh and eighth goals of the post-season, including a redirection with 5:48 left to trim Anaheim's lead to one goal.
Los Angeles pressed throughout the final minutes with a playoff-high 42 shots, but couldn't beat Gibson for the equalizer.
Trevor Lewis also scored and Jonathan Quick stopped 20 shots for the Kings, who are on their second three-game skid of the post-season — bookending a six-game winning streak
The road team had won every game in the first series between these rivals separated by 30 miles of freeway, but the Ducks snapped the Kings' four-game road playoff winning streak dating to their 0-3 comeback in the first round against San Jose.