WhatsApp is now rolling out an update for the Android and iOS versions of the app that adds document sharing. More specifically, PDF sharing.
You can now press the attachment sharing button to see the option to attach documents. On Android it will show your file manager to attach any local PDF files you may have. On iPhone, it will let you attach files from other storage apps, such as Dropbox, Google Drive, and OneDrive.
The update is now rolling out and should be available to everyone soon.