The actual font data is missing from the file you are trying to view, and your computer doesn't know which "real" font it's supposed to map to.
Elias stared at the screen. The font hadn't just fixed the diagram; it had decoded the truth.
When a PDF is created, the exporting software (like Adobe InDesign or a printer driver) often renames embedded fonts to CIDFont+F1 , F2 , F3 , etc., to handle large character sets (often Asian or technical characters).