In this deep-dive article, we will explore the architecture of CID-keyed fonts, decode the meaning of F1 through F4, diagnose common rendering failures, and provide a definitive guide to achieving performance, file size, and visual fidelity. What Are CID Fonts? A Brief Primer Before we can understand why "F1, F2, F3, F4 better" matters, we must understand CID (Character Identifier) fonts.
Use Adobe-Japan1 , Adobe-GB1 (Chinese), or Adobe-Korea1 CMAPs explicitly. Avoid generic Identity unless you control the mapping end-to-end. 5. Repair Broken F1/F2/F3/F4 with Ghostscript or qpdf When you encounter a PDF that shows garbled text due to bad CID labels, use Ghostscript to rewrite the font structure: cid font f1 f2 f3 f4 better
Unlike simple fonts (Type 1 or TrueType) that map a single byte to a glyph, CID fonts are designed for large character sets. A CID font separates the (the set of glyphs) from the CMAP (character map). The PDF specification uses numeric labels—often F1, F2, F3, F4 —as font aliases or internal names for these CID-keyed fonts when the original font name is missing or when subsetting occurs. The Role of F1, F2, F3, F4 in PDF Structure When you extract a PDF’s font dictionary, you might see: In this deep-dive article, we will explore the
/F1 /CIDFontType0
import fitz # PyMuPDF doc = fitz.open("bad_fonts.pdf") for page in doc: for block in page.get_text("dict")["blocks"]: for line in block["lines"]: for span in line["spans"]: if span["font"].startswith(("F1","F2","F3","F4")): print(f"Found CID alias span['font'] at span['bbox']") # Fix: Re-encode page or extract text manually doc.close() Repair Broken F1/F2/F3/F4 with Ghostscript or qpdf When
pdffonts yourfile.pdf Look for the "Type" column: CIDFontType0 or CIDFontType2 . Then inspect the "CMAP" column. If you see Identity-H but the language is Japanese, no direct conversion is possible without a custom CMAP.