When a PDF is created, each font used is assigned a —F1, F2, F3, etc. If the original font is missing on your system, the PDF viewer or RIP (Raster Image Processor) substitutes a generic CID font but retains the label. Over time, users began searching for actual font files named "F1.ttf" or "CID-F1.otf" to prevent substitution errors.
CID (Character IDentifier) is a technology used in PDFs to handle large and complex character sets, particularly for East Asian languages like Chinese, Japanese, and Korean, or for large Unicode character sets. cid font f1 f2 f3 f4 f5 f6 f7 fonts free download new
You can also create a test PDF: