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pdflatex.log
Your document uses ctex (\documentclass{ctexart}, ctexbook, ctexrep, ctexbeamer, or \usepackage{ctex}) and it was compiled with pdfLaTeX. Compile it with XeLaTeX instead. The document itself does not need to change. The font number and size in the message (unisong30 at 632 above) vary with the document and its font size. Every variant has the same cause and the same fix.

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Why pdfLaTeX fails

ctex loads its Chinese fonts from a font set. In TeX Live 2026 on Linux, which is the environment the examples on this page are compiled in, ctex selects the Fandol font set. That font set needs XeLaTeX or LuaLaTeX, so under pdfLaTeX ctex reports CTeX fontset `fandol' is unavailable in current mode and falls back to the old unisong font names of the CJK package. TeX Live does not include those fonts, so pdfTeX stops with a fatal error and writes no PDF. The first message is the cause. The unisong message is the consequence, and it is the one most logs show last.

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Minimal failing example

ctex-pdflatex.tex

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Fix: compile with XeLaTeX

Change the compiler, not the document. The same source compiles with XeLaTeX:
ctex-xelatex.tex
Ready to use this syntax? Continue in the browser editor when you want to adapt the example in a real project.Open LaTeX Cloud Studio
On the command line, run xelatex main.tex or latexmk -xelatex main.tex instead of pdflatex main.tex. LuaLaTeX (lualatex main.tex) also compiles this document.

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If you must stay on pdfLaTeX

Some journal or university templates only build with pdfLaTeX. In that case, do not load ctex. Use the CJKutf8 package with a font that TeX Live ships, such as gbsn (Simplified Chinese, Song style), gkai (Simplified Chinese, Kai style), or bsmi (Traditional Chinese):
cjkutf8-pdflatex.tex
This only typesets the characters. ctex features such as Chinese captions (目录, 参考文献) and the \zihao font-size command are not available, so prefer XeLaTeX whenever the template allows it.

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The same error with the CJK package

\begin{CJK}{UTF8}{song} fails under pdfLaTeX in the same way, with a different font name:
pdflatex.log
Replace song with gbsn, or switch to XeLaTeX and ctex.

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In LaTeX Cloud Studio

  • The project’s LaTeX Engine setting defaults to Auto (Recommended). With Auto, a document that uses ctex, ctexart, ctexbook, ctexrep, ctexbeamer, xeCJK, fontspec, or polyglossia, or calls \setCJKmainfont, compiles with XeLaTeX. Packages loaded from your own .sty and .cls files count too, and commented-out lines are ignored.
  • Documents that use luatexja or the ltjs* classes for Japanese compile with LuaLaTeX.
  • If the engine is set to PDFLaTeX explicitly, that choice is kept and the compile fails. The Problems panel then shows Chinese font unisong30 is not available for pdfLaTeX. Open the project Settings, and under General set LaTeX Engine to Auto (Recommended) or XeLaTeX.