I don’t think it was PR, but I do believe that he was surprised by the success of his bid.
Initially, I think Trump thought it was a good way to gain more influence and attention while sticking his fingers in the eyes of the political powers that be. It was his own, wholly owned, reality TV show.
Along about the ‘shoot somebody in the middle of 5th Avenue’ comment, it dawned on him that people were taking him seriously. VERY seriously.
The fact that he had very little ground organization and was blindsided by the super-delegates and delegate thievery of Cruz points to the fact that it was not a real bid in the beginning. Trump is a good manager. If he’d planned a real bid from the outset, he would have hired insiders that knew how the dirty political games are played.
And probably lost.