This typically 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
- Select Save As after changing your global SAVE properties to embed the fonts
- Select PDF as the output.
This should embed the fonts properly, so they will appear as you've designed them once uploaded to your lesson.
Learn more about Embedding Fonts.