Quote's PDF can be modified, and this is how you can do it:
- Click on Quote submenu from CRM module.
- Click on
the click on Quotes Settings. 
- Click on Advanced Settings then select your Business Unit.
- Click on Modify button of wich part of PDF you want to edit.
- From here you can select Label or Variable and add text to the PDF. After all of this click on Save.
Modified PDf will appear even if you sent it by email or just download it.