Some PDFs do not reference the full CID font; they embed only the used glyphs. If the subset is malformed, no external font helps.

Avoid random "free font" sites offering cidfont_f1.exe or F1.otf – those are often:

Google Noto Fonts – 100% free, open-source.

Use this checklist to resolve the missing "CID font F1" error for good.