I suspect the people making the lectures are using a font which is not available on the iPad, and users cannot add fonts, so there is no way to fix it.
The symbol you describe is probably this one from Apple's last resort font, which is displayed when no font has a character from the Private Use Area of Unicode. The Private Use Area should not be used when you are creating text that others have to be able to read. I am sure they could find a standard Unicode character to replace what they are currently using. If you can provide a photo (use the camera icon to upload), I could probably suggest one.