To attach documents to an email, please follow the below steps:
Create a new email.
Click the Attachments tab at the top of the window.
Under File attachments click Add.
Navigate to the file you wish to attach, click on it and click Open.
The file is now attached to the email and you can proceed with sending the email.
If the file you wish to attach needs to be merged from the record you are emailing, please see the knowledge article Notebook Types and Templates.