Search PDF File Through Google Search

You can search any formats of file by google advance search (進階搜尋, e.g.
In Chinese
http://www.google.com.tw/advanced_search?q=Burdock+filetype:pdf&hl=zh-TW&lr=lang_en&client=firefox-a&hs=SQh&rls=org.mozilla:en-GB:official&as_qdr=all&tbs=lr:lang_1en&prmd=imvns
In English
https://www.google.com/advanced_search?q=Burdock+filetype:pdf&hl=en&lr=lang_en&client=firefox-a&rls=org.mozilla:en-GB:official&as_qdr=all&tbs=lr:lang_1en&prmd=imvns)

You may want to use this way to search FREE PDF articles, FREE articles from journals or even handbooks.