There is exactly one question that decides the format: is the person receiving this going to stand up and present it, or read it at their desk? Everything else follows.
They are going to read it
Send a PDF. It opens everywhere, on every device, with the fonts you chose and the layout you built, and it cannot be accidentally re-animated or half-edited. Ninety per cent of decks that get emailed are read rather than presented, and almost all of them should have been PDFs.
They are going to present it
Send PPTX, and ask which version of PowerPoint they have. If the answer involves a conference laptop, an old projector, or the phrase “the IT people set it up”, convert to PPT as well and send both. It costs you nothing and saves the five minutes before a talk that nobody has.
- Reading it: PDF.
- Presenting it: PPTX, plus PPT if the machine is unknown.
- Putting slides in a document or a post: PowerPoint to JPEG, one image per slide.
- Reusing the words as prose: PowerPoint to Word, then expect to rewrite.
Worth knowing
Fonts are the usual failure. If the deck uses something you installed yourself, it will substitute on their machine and your careful line breaks will move. A PDF removes the problem entirely.