YSK: adding "filetype:pdf" to your google search query will make all results link to pdf documents. You can often use this to find good and full explanations of practical concepts in your field.
from /r/YouShouldKnow
YSK：在你的谷歌搜索查詢中加入 "filetype:pdf"，會使所有結果都鏈接到pdf文件。你經常可以用它來尋找你所在領域的實用概念的良好和完整的解釋。