I have a bunch of course material in .ppt and .pptx format, but I don't have and don't want OpenOffice installed. Since I'm using Linux, Office also is ruled out.
What can I do to convert them to a PDF format with the best fidelity available?
PDF Converter does exactly what you're looking for.
You can upload any file to convert to pdf for free. If you have to send more than one file, simply zip them all, send it as a zip file and when everything is ready, they will notify you via e-mail.
More of a SuperUser answer than a WebApps answer, but Microsoft's free Powerpoint viewer app apparently works in WINE under Linux, it should have a print option that you can then point to your PDF printer, have you tried that?
A non fully web apps answer would be: