备注pdfdist的ReadMe

public/pdfjs-dist copy/README.md

 # PDF.js
 
+高版本的pdfjs，暂不兼容低版本浏览器
+
 PDF.js is a Portable Document Format (PDF) library that is built with HTML5.
 Our goal is to create a general-purpose, web standards-based platform for
 parsing and rendering PDFs.

public/pdfjs-dist/README.md

 # PDF.js
 
+这个包是低版本的pdfjs，兼容低版本浏览器，但是添加笔记的逻辑暂未添加进去
+
 PDF.js is a Portable Document Format (PDF) library that is built with HTML5.
 Our goal is to create a general-purpose, web standards-based platform for
 parsing and rendering PDFs.