Is this discouraged? Say I wrote an essay of 20 pages and I'd like to present it in a power point show. Can I simply copy paste what I already wrote in the essay (with shortening it, of course) without the need to re-paraphrase again?
Please, please, please, for the sake of your audience do not create a talk simply by dumping a giant pile of text onto slides. Prose is dense and meant to be digested slowly and without aid from the author. A talk is just that: a talk by the author, and any slides should be there simply to enhance the audience's understanding of your words.
Thus, your slides should generally be light on words and heavy on images and diagrams. You can dump text into the "notes" section that only you the presenter will see, but if you dump it on screen for the audience then 1) they will be reading the text rather than listening to you, and 2) they will be terribly bored and think you are a terrible presenter.
For some better heuristics for approaching the constructions of a presentations, I recommend watching Patrick Winston's "How To Speak" talk.