There's two things.

LLMs can use PowerPoint, but also LLMs can write the raw document file directly without the software. One time I asked for a letter and what came out was it doing that.

What really matters with this though is reading other people's documents, whatever format they happen to be in. PowerPoint isn't the only game in town for creating slide decks, but if you want to be sure you can read all the ones sent to you, it used to be the case you'd need a copy.

Used to be. Now the AI can read and summarise the slides for you.

("AI, expand these bullet points into a slide deck" -> "AI, condense this slide deck into bullet points", as the meme goes).