A free PowerPoint converter: open a PowerPoint or LibreOffice deck, see every slide as it will be written, and save it as any of 13 formats. Each result is opened again and compared with your deck. Nothing is uploaded.
A .pptx names its fonts but rarely contains them. Each one on the right is tested by drawing the same text with it and with three fallback families; if every width matches a fallback, the font isn't installed and PowerPoint would substitute it.
One 11-slide deck (929.8 KB PPTX with titles, bullets, a data table, coloured shapes and two photographs) converted into every format and measured. Measured with LibreOffice; your own files will differ, especially with more photos or video, but the order holds.
The test takes the deck above, writes it in all 13 formats this page saves, opens every result again and compares slides, words, pictures, tables, notes and links with the original. The binary 97-2003 formats and Keynote go to the tools built for them.
What each one is for, what changes on the way, and one thing worth checking before you send it.