What are my options to embed private PowerPoint presentations on Plone site
Presentations would be behind Plone log-in
Integrate some sort of presentation viewer which can input PPT presentations
File converter (thru PDF?) or external service (slideshare) can be used, as long as data stays private
In the worst case is to have MS Office scripting which exports presentations as PNGs, but still one would need the viewer library.
You could also use some external services, like issuue.
(I made an add on that supports that, but since issuue api is not free anymore, it is kind of discontinued (if you already have a issue login it might work with 'free version').
If the PowerPoint documents are fairly simple, you might be able to use "LibreOffice" (via "pyuno") to convert them to PDF.
Export your files to PDF which is supported by any application.
is the best tool for converting PDF to exact and searchable HTML.
...and perhaps forget about the other tinkered workflows.