Can I send a document along with my WhatsApp message?

Yes, you can.
Let us consider the following case. Have you ever booked a ticket on a travel or ticketing portal? On booking a ticket, you will have received a confirmation message on your WhatsApp number along with a PDF form of a ticket.

Similarly, you can attach and send various documents like invoices, quotes, brochures, etc,. along with your WhatsApp messages.