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：在你的谷歌搜索查询中加入 "filetype:pdf"，会使所有结果都链接到pdf文件。你经常可以用它来寻找你所在领域的实用概念的良好和完整的解释。