Actually, when asked to order something, he first asks for a Tab . . . but of course, back in 1955, "tab" meant a running bill; there was not yet a "Tab" product (from Coca-Cola). When told he can't be given a tab until he orders something, he then asks for a "Pepsi Free" (which was the name of Pepsi's diet cola in 1985) . . . but of course, no soda shop owner is going to give him a soda for free! Finally he asks for "something without any sugar in it." Given the way that scene played out, I think the then-current (i.e., 1985) name of the "Pepsi Free" product may have had something to do with the choice of Pepsi as the drink. Note also that there was a nearly concurrent marketing campaign featuring Michael J. Fox in commercials for Pepsi, "The choice of a new generation." Certainly they were capitalizing on the popularity of the film, and I wonder if this was an arrangement that was part of the product placement deal between the studio and Pepsi; generally, when a product is featured in a movie, the producers of the film try to get the brand to pay, because of the exposure they'll get.