S.E. Hinton's novel The Outsiders is set in 1967. Pepsi was only sold in glass bottles in those days. The glass bottles that contain Pepsi are used as weapons a couple of times in the novel when someone breaks a piece off a bottle and threatens another with it. So it could be argued that Pepsi has a symbolic connection to violence.
A better connection to the symbolism of Pepsi in the novel is the loss of innocence it represents. Ponyboy talks about Pepsi when he is hiding out in the church with Johnny after the murder:
"By the fifth day I was so tired of baloney I nearly got sick every time I looked at it. We had eaten all our candy bars in the first two days. I was dying for a Pepsi. I'm what you might call a Pepsi addict. I drink them like a fiend, and going for five days without one was about to kill me. Johnny promised to get some if we ran out of supplies and had to get some more, but that didn't help me right then."
Ponyboy and Johnny never had a simple life, but they had a life they were comfortable with. When Ponyboy writes about his withdrawal from Pepsi, it represents the loss of life as they knew it. Pepsi was part of his everyday routine, and now that routine is gone. Life for Ponyboy and Johnny will never go back to the way it was before the murder.
They have been in isolation for five days at this point in the novel. The weight and reality of what has happened has settled in and they have no idea what their future will hold. Not only is Ponyboy physically pining for Pepsi, he is also pining for his life before hiding in the church. Pepsi, which was so much a part of everyday life for him, is now part of that desire to return to life the way it was.