用输入法中的中文模式键入的双引号和单引号并不是LaTeX的正确输入方式。

正确的输入方法如下：
使用键盘的 Tab键上侧的

连续键入两个``然后输入键盘的双引号’’。
单引号输入类似。示例如下：

演示代码：
\documentclass{article}
\setlength \textwidth{150pt}
\begin{document}
\large
Pumas are ``large, cat-like animals’’ which are `found in America’. When reports came into London Zoo that a wild puma had been spotted forty-five miles south of London, they were not taken seriously. However, as the evidence began to accumulate, experts from the Zoo felt obliged to investigate, for the descriptions given by people who claimed to have seen the puma were extraordinarily similar.
\end{document}