Older versions of PDF viewers do not always have the necessary CMap libraries to decode modern CID-keyed fonts. When they encounter an advanced multi-byte structure labeled as "F1," they fail to parse the data correctly. How to Fix CID Font F1 Issues
The "F1" designation is simply an internal variable name or shorthand alias created by the PDF authoring software (such as Adobe Acrobat, Microsoft Word, or InDesign). cid font f1 family