1. Add _posts/pdf_posts_to_json.py
	- glob the pdf files in `pdf_posts`
	- write the file-list in json form to _data/pdf_posts.yml

2. _data/pdf_posts.yml
	- data file record all the pdf files in `pdf_posts` dir
	- refereced by index.html

3. index.html
	- Add the part of inserting the content in pdf_posts.yml

NOTE:

pdf_posts.yml is JSON file though the extension is `yml`.
Why? Because the jekyll just ignore the *.json while process json file
with yml extension. FUNNY? I guess this was caused by the fact that:
JSON is a subset of YAML.