This article covers
How to make the coupon and discounts show on the left side grey box in the quote PDF?
Whenever the coupon is applied to a specific line item then the coupon will be displayed on the left bottom box.
Create a coupon and apply on specified line item set on specific plan
Create a quote for a new subscription and add plan and a coupon to it
view the PDF you will see the discount in the left bottom box.
Here's a video guide for the same. Sample quote and the quote PDF