使用R Markdown与LaTeX制作论文模板时遇Missing \begin{document}错误求助

问题描述 #

编译基于R Markdown和LaTeX的论文模板时，出现如下错误：

! LaTeX Error: Missing \begin{document}.
Error: LaTeX failed to compile paper_template.tex

用户提供的代码如下：

R Markdown 文件代码 #

---
title: "Paper Template"
author: "AAA"
abstract: ""
output:
  pdf_document:
    latex_engine: xelatex
    includes:
      in_header: customized_paper_template.tex
date: "2024-06-29"
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)

\newpage

R Markdown #

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see http://rmarkdown.rstudio.com. When you click the Knit button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:

（重复内容省略）

### customized_paper_template.tex 文件代码
```latex
\usepackage{fontspec}
\usepackage{geometry}
\geometry{a4paper, margin=1in}
\usepackage{setspace}
\renewcommand{\baselinestretch}{1.25}

% Set body text font
\setmainfont{Kepler Std Light Semicondensed}

% Set title fonts
\newfontfamily\titlefont{Tiempos Headline}

% Set the font size for the document
\AtBeginDocument{\fontsize{12pt}{14pt}\selectfont}

% Define title formatting
\usepackage{titlesec}
 \titleformat{\section}
  {\normalfont\LARGE\titlefont} % Title style
  {\thesection} % Section number
  {1em} % Space between number and title
  {} % No additional formatting before the title text
   [\titlerule[0.5pt]] % Add a line under the title with 0.5pt thickness

\titleformat{\subsection}
  {\normalfont\Large\titlefont} % Title style
  {\thesubsection} % Section number
  {1em} % Space between number and title
  {} % No additional formatting before the title text
  [\titlerule[0.4pt]] % Add a line under the title with 0.4pt thickness

\titleformat{\subsubsection}
  {\normalfont\normalsize\titlefont} % Title style
  {\thesubsubsection} % Section number
  {1em} % Space between number and title
  {} % No additional formatting before the title text
  [\titlerule[0.3pt]] % Add a line under the title with 0.3pt thickness

\titleformat{\paragraph}
  {\normalfont\normalsize\titlefont} % Title style
  {\theparagraph} % Section number
  {1em} % Space between number and title
  {} % No additional formatting before the title text
  [\titlerule[0.2pt]] % Add a line under the title with 0.2pt thickness

\titleformat{\subparagraph}
  {\normalfont\normalsize\titlefont} % Title style
  {\thesubparagraph} % Section number
  {1em} % Space between number and title
  {} % No additional formatting before the title text
  [\titlerule[0.2pt]] % Add a line under the title with 0.2pt thickness

错误原因 #

这个报错并非真的缺少\begin{document}，而是前置代码（preamble）出现错误导致编译中断：

1. 字体缺失：Kepler Std Light Semicondensed和Tiempos Headline是商业字体，系统默认未安装，fontspec无法找到字体时会触发编译错误。
2. 字体大小设置时机冲突：\AtBeginDocument的代码会被插入到\begin{document}之后，但R Markdown的LaTeX模板已经有自己的字体大小设置，可能引发冲突。

解决方案 #

1. 替换为系统自带字体 #

将自定义字体替换为系统内置字体，比如：

• 英文环境：Latin Modern Roman、Times New Roman
• 中文环境：SimSun（宋体）、Microsoft YaHei（微软雅黑）

2. 调整字体大小设置方式 #

直接在preamble中设置字体大小，无需使用\AtBeginDocument。

修改后的customized_paper_template.tex代码 #

\usepackage{fontspec}
\usepackage{geometry}
\geometry{a4paper, margin=1in}
\usepackage{setspace}
\renewcommand{\baselinestretch}{1.25}

% 替换为系统自带字体
\setmainfont{Latin Modern Roman}
% 标题字体也替换为系统可用字体
\newfontfamily\titlefont{Latin Modern Sans DemiCondensed}

% 直接设置字体大小
\fontsize{12pt}{14pt}\selectfont

% 标题格式定义
\usepackage{titlesec}
 \titleformat{\section}
  {\normalfont\LARGE\titlefont}
  {\thesection}
  {1em}
  {}
   [\titlerule[0.5pt]]

\titleformat{\subsection}
  {\normalfont\Large\titlefont}
  {\thesubsection}
  {1em}
  {}
  [\titlerule[0.4pt]]

\titleformat{\subsubsection}
  {\normalfont\normalsize\titlefont}
  {\thesubsubsection}
  {1em}
  {}
  [\titlerule[0.3pt]]

\titleformat{\paragraph}
  {\normalfont\normalsize\titlefont}
  {\theparagraph}
  {1em}
  {}
  [\titlerule[0.2pt]]

\titleformat{\subparagraph}
  {\normalfont\normalsize\titlefont}
  {\thesubparagraph}
  {1em}
  {}
  [\titlerule[0.2pt]]

额外检查项 #

• 确保已安装titlesec宏包（TeX Live或MikTeX默认包含）。
• 若需使用自定义商业字体，需将字体文件放入系统字体目录，或通过Path参数指定字体路径，例如：

  \setmainfont[Path=/path/to/fonts/]{KeplerStd-LightSemicondensed.otf}
  

内容的提问来源于stack exchange，提问作者mzkrc