How can I view and edit PowerPoint presentations that don't render correctly?

Actually, it may be font issues. Because of Microsoft's fonts are not included in the Linux OS, system substitute with specific fonts.

But these font may not be in same grid size or font attributes.

Try to install Microsoft fonts with following commands in your terminal.

wget http://httpredir.debian.org/debian/pool/contrib/m/msttcorefonts/ttf-mscorefonts-installer_3.6_all.deb
sudo dpkg -i ttf-mscorefonts-installer_3.6_all.deb

Check your documents again. And you should also check fonts such as Arial, Calibri, etc. in font list.

If you have a licensed copy of PowerPoint, you can install it in Ubuntu using Wine from the default Ubuntu repositories. Wine is a compatibility layer for running Windows applications on Linux. You will also need to install Installer for Microsoft TrueType core fonts (ttf-mscorefonts-installer).

If you don't have a licensed copy of PowerPoint, you can download the free PowerPoint Viewer from the official Microsoft Download Center and use it for viewing PowerPoint presentations or you can upload an incompatible PowerPoint presentation to Google Drive, open it with Google Slides and select File -> Download as to convert it to a format that is compatible with Impress and download it to your computer.

Most of the times, this issue pops up due to the standard fonts that come with Windows and are not available in Linux due to license or other issues. As a workaround, I usually create a .fonts folder in the home directory (if it doesn't exist) and then copy them there from a Windows fonts library.

Then manually rebuild the font cache:

fc-cache -f -v

However, this is a workaround and I will not recommend this for the obvious license issues.