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Some packages and fonts require a specific engine.

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Common Symptoms

  • Unicode glyph failures under pdfLaTeX.
  • fontspec errors with pdfLaTeX.
  • CJK or RTL rendering issues.

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The exact error you probably searched for

fontspec-pdftex.log
{cannot-use-pdftex} means exactly one thing: the document loads fontspec, which needs system fonts, but it is being compiled with pdfLaTeX, which cannot use them. Switch the compiler to XeLaTeX or LuaLaTeX. Nothing in the document needs to change. If you cannot switch, remove fontspec and select fonts with a pdfLaTeX package such as \usepackage{times} or \usepackage{newtxtext} instead. In LaTeX Cloud Studio the compiler is a project setting, so this is a one-click fix rather than a rewrite.

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Rule of Thumb

  • pdfLaTeX: classic latin workflows.
  • XeLaTeX/LuaLaTeX: modern Unicode, system fonts, CJK, RTL.

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Example