The pdf of Sang Lei, the first lesson of law, can be obtained from some online bookstores or law libraries. Here are some possible ways to get it:

1. Online Bookstore: You can buy e-books or paper books in online bookstores such as Amazon, Dangdang and JD.COM, which usually provide free PDF downloads.

2. Law Library: If you are in the library of a university or law school, you can try to find the PDF version of the book in the law library.

3. Public Library: If you are in a public library in a city or region, you can also try to find a PDF version of the book there.

Whichever way you choose, make sure you get a legal and authentic PDF version.