Solution 1:

ONLYOFFICE claims to be higly compatible with MS Office formats, and it is released under AGPL. There are many ways to install it in Ubuntu, use your favourite method.

It is a bit slow to start (this is not a snap only issue, even the .deb package takes a lot of time to start compared to libreoffice), but successfully loads annotated powerpoint presentations (verified by OP).

Snap Package

ONLYOFFICE is available as a snap and can be installed with

snap install onlyoffice-desktopeditors

Debian package

A .deb-package is also available.

Flatpak

Use the following commands to install it with Flatpak.

flatpak install flathub org.onlyoffice.desktopeditors

Disclaimer: I'm not affiliated with ONLYOFFICE in any way.

Solution 2:

The answers will not be to your liking:

  1. Nope.
  2. They can be viewed with O365’s online version of PowerPoint for “free”, but not as free as you are asking