This very much depends on what you would like to do in the system whether it is sending an attachment to one customer or more.
So if you are looking to sending a quick quote or information to an individual customer you can do this by sending a Plain Text Email through the contact record or if it's through a bulk email campaign you will need to add a document link.
Here are other relevant guides:
[Video] How to add a document to an email campaign
Linking to Documents in an email campaign