While I cannot provide a free PDF, you can legally access the Longman Language Activator in these ways:
: It explains the subtle differences between similar words (e.g., the difference between scorching , stifling , and humid ).
When learning English, most students rely heavily on a standard dictionary. You look up a word (e.g., walk ), find the definition, and move on. But what happens when you want to say walk but you mean stroll , strut , wander , or march ? A standard dictionary fails you here because it assumes you already know the word.