Converting PPT and PPTX slideshows to PDF is not built yet. Rendering slides faithfully means handling slide masters, embedded fonts, themes and layout inheritance — reproducing that in the browser without a real rendering engine for the format would produce output too unreliable to ship.

Until then, exporting to PDF directly from PowerPoint, Keynote or Google Slides gives an accurate result. Once you have a PDF, every other tool here works on it normally.