When software generates a PDF, it frequently creates internal aliases for the fonts used in the document to compress file size and streamline rendering.
What or character set is the document supposed to display?
If you have Acrobat Pro, you can try to force the document to recognize the fonts: Tools > Print Production > Preflight Select the "PDF Fixups" "Embed missing fonts" Analyze and Fix 3. Manual Substitution If you are trying to edit the text in a program like Adobe Illustrator tool (Type > Find Font).
Understanding CID Fonts (F1, F2, F3, F4, F5, F6, F7) What Are CID Fonts?
If the names are gone, the fonts are often standard system fonts like Times New Roman 2. The "Preflight" Fix (Acrobat Pro)