Can I present a .pptx without PowerPoint installed?

Yes. PowerPoint Online (office.com), Google Slides (slides.google.com) and Keynote (Mac) all present .pptx in full-screen mode. For one-shot presentations on someone else's computer, exporting to PDF is the safest bet.

Do animations and transitions survive?

Standard slide transitions (fade, dissolve) and basic animations (appear, emphasis) survive in PowerPoint Online and largely survive in Google Slides. Complex morph transitions and timed sequences sometimes break in Google Slides; PowerPoint Online is more faithful.

What about embedded fonts?

School computers often have fonts your home computer doesn't. If your deck looks subtly different at home, that's the cause. PowerPoint Online substitutes missing fonts to the nearest match; Google Slides does the same. Embedding fonts in the .pptx before exporting (Save As → Tools → Save options → Embed fonts) avoids this.

File types · PowerPoint

.pptx files, still presentable.

The decks you made for school presentations still open everywhere, free.

Free options

PowerPoint Online — office.com with a free Microsoft account. Closest to the original.
Google Slides — drive.google.com → upload → Open with Slides. Converts on import.
LibreOffice Impress — desktop, offline, free.
Keynote — on Mac, opens .pptx natively.

For one-off presentations

If you're presenting on someone else's computer (a university lecture theatre, an interview room), export the deck to PDF first and bring the PDF on a USB. PDFs open anywhere, render identically everywhere, and never have a missing-font problem.

Common questions

Can I present a .pptx without PowerPoint installed?: Yes. PowerPoint Online (office.com), Google Slides (slides.google.com) and Keynote (Mac) all present .pptx in full-screen mode. For one-shot presentations on someone else's computer, exporting to PDF is the safest bet.
Do animations and transitions survive?: Standard slide transitions (fade, dissolve) and basic animations (appear, emphasis) survive in PowerPoint Online and largely survive in Google Slides. Complex morph transitions and timed sequences sometimes break in Google Slides; PowerPoint Online is more faithful.
What about embedded fonts?: School computers often have fonts your home computer doesn't. If your deck looks subtly different at home, that's the cause. PowerPoint Online substitutes missing fonts to the nearest match; Google Slides does the same. Embedding fonts in the .pptx before exporting (Save As → Tools → Save options → Embed fonts) avoids this.