Update README.md

Author: michaelliao

README.md

@@ -4,6 +4,8 @@ Source code and markdown documents of [https://liaoxuefeng.com](https://liaoxuef
 
 This website is generated by [GitSite](https://gitsite.org) and served as static website.
 
+PDF can be downloaded from [https://dl.lxf.sh/pdf/](https://dl.lxf.sh/pdf/).
+
 ## Download
 
 You can download all markdown documents and generated static web site from [Release](https://github.com/michaelliao/liaoxuefeng.com/releases).