Franklin's writing style is plain and pragmatic, almost journalistic. When describing the events he includes in the Autobiography, he's direct about what happens to him and how he feels about it. If we were living in 1786, we'd probably understand him perfectly. Franklin's committed to involving readers in his text, almost like he's talking directly to us. For example, he opens his autobiography by saying:
Now imagining it may be equally agreeable to you to know the Circumstances of my Life, many of which you are yet unacquainted with; and expecting a Week's uninterrupted Leisure in my present Country Retirement, I sit down to write them for you. (1.1)
Although Franklin's supposedly addressing his son here, when he says "you," he's also talking directly to us, his general audience. Franklin's clear and realistic here. He explains to us why he's making the choice of writing an autobiography, by saying he thinks that it would be fun for us to learn about some secret parts of his life we don't know yet. He also tells us why he's got time to do this writing. After all, he's a busy guy; maybe he should be out in the world gathering more experiences to put into his autobiography instead of just sitting down and writing it. Instead, he tells us he's got some time on his hands, and he wants to use it semi-productively by writing down his life story. This also tells us something about Franklin's character: even in his moments of leisure and relaxation, when he'd be totally justified in getting a tan and drinking mai tais, he doesn't rest on his laurels. Instead, he describes them.