我能弄明白。嵌入标记不适用于pdf，因为它不通过浏览器渲染。使用对象标记有效：

<object data=\'http://example.com/wp-content/uploads/...somefile.pdf\' type="application/pdf" 

width="1000px" height="1200px">

<p>It appears you don\'t have a PDF plugin for this browser. You can download the pdf
<a href=\'http://example.com/wp-content/uploads/.../somefile.pdf\'>here</a>
</p>

</object>