If you are currently trying to fix a specific file, please let me know: What you used to open or create the PDF? Whether you need to edit the text or just print/view it?
A: The original font license was likely restricted. The PDF creator tool substituted the F1 fallback to avoid legal distribution of a commercial font. cid font f1 family
For graphic designers experiencing pre-press errors, converting text layers into physical vector paths in Adobe InDesign or Illustrator removes the font registry requirement completely. If you are currently trying to fix a