Open Make PDF smaller from the Home screen (or use its own page link).
Tap Choose file and select your PDF from the phone or computer.
Choose WhatsApp.
Tap the big purple Start button and wait a few seconds.
Check the result line, it shows the old and new size (for example 4 KB → 3 KB). If the text looks blurry, go back and choose a bigger size.
Want to send it right away? Tap Share / WhatsApp instead.