Create presentation from file starts the deck from content you have already written. In Copilot in PowerPoint, you reach it from the Copilot button on the Home tab and point it at a Word document, or at a PDF if you have a Microsoft 365 Copilot work licence. Copilot splits the document into slides, applies layouts, writes speaker notes and picks a theme. It reads Word's heading styles to decide where one slide ends and the next begins, and images already in the document can carry over.
You cannot add extra instructions in the same prompt, so "keep it under eight slides" has to wait: build from the file first, then edit the result. You also need access to the file, and Copilot works best with documents under 24 MB. Copilot Pro offers a more limited version. Review the draft before you share it, as with any Copilot output.