Many PDFs are created without fonts fully embedded. The creator may have opted to save file size by not including the complete font data. When you download such a PDF, your local viewer has no source to pull from.
This message appears when a software application (like Adobe InDesign, Microsoft Word, or a PDF reader) opens a file that calls for a specific font not currently installed on your computer's operating system. Download Font Substitution Will Occur
"Updating my PDF viewer will fix the warning." Fact: No. Updates add features, not missing fonts from the original creator. Many PDFs are created without fonts fully embedded