This generally happens if you are using a non-web safe font and haven't properly embedded them when saving in PowerPoint.
To fix this, you'll need to re-save your PDF file with the fonts embedded.
In PowerPoint, click on File > Save As after changing your global SAVE properties to embed the fonts, then select PDF as the output. This should embed the fonts properly so they will appear as you've designed them to appear once uploaded to your lesson.
Learn more about embedding fonts in PowerPoint here: Embedding Fonts