WhatsApp doesn't supporting any of erternal documents. It only supports images videos and audio only. We can only send documents trough watsapp. To read it you need external application that can read PDF . There is lots of PDF reader in play store and app store . AND YES IT IS A SAFE WAY TO TRANSFER A FILE. IF PDF IS PASSWORD PROTECTED NO ONE WITHOUT PASSWORD CANT ACCESS IT. AND ANOTHER BENEFIT IS WHEN YOU SEND A IMAGE OR ANY OTHER FILE AS DOCUMENT IT DIDN'T LOOSE ANY DATA OR QUALITY.