1. Mark "Send Products Attachments" on mail tempalte (By default it will be done.)
2. Add documents on Products which you want to send in mail.
Odoo Mail Product Documents
Just install this module to add functionality. Not any extra configurations required.
Mail Product Documents with Sale Order Mail Automatically in odoo
Send Product Images, Brochure or any other Attachments automatically related to your products when you send proposal by mail