You need it for a graded course, or you care about the absolute latest research in computational linguistics or psycholinguistics.

Yule’s 8th edition is a masterpiece of clarity. But the information inside is not a secret. The core concepts of linguistics haven't changed dramatically in the last five years. If you can't find the PDF legally, grab a cheap used 7th edition for $8 on AbeBooks. You will learn 99% of the same material and save your hard drive from malware. Have you found a reliable source for Yule’s 8th edition, or do you prefer the physical copy? Let us know in the comments below.

You are an independent learner who just wants to understand why "Buffalo buffalo buffalo" is a real sentence. For casual curiosity, the 8th edition PDF (if you find a clean copy) is fine.