Many learners still prefer the tactile feel of paper. A PDF allows you to print specific "word blocks" to highlight, scribble notes, or pin to your fridge.
Unlike apps that require data or a subscription, a "portable" PDF lives on your device. You can study on a plane, a train, or in a remote area without Wi-Fi.
The English language has over 170,000 words in current use, but the average native speaker uses only a fraction of them in daily life.
