You use: t-esc

<span t-esc="l.price_unit*(1-l.discount)"/>

It works, thank you!

Just note for others: value of discount is not stored in %, so it should be like this  

<span t-esc="l.price_unit*(1-l.discount/100)"/>

Thank you !! it worked wonderfully.

For those who are novices with odoo like me here's a step by step on how to add it. Using odoo 11.0

1) go to settings and activate developer mode > then reports > reports https://i.imgur.com/uJactI5.jpg

2) after that on the search bar type "sale.order" and then click on the model "sale.report_saleorder"
3)click on qweb view, it has the icon " </> "

4) Click on "report_saleorder_document" and then modify.
5) search for this and add this line,you can name it anything https://i.imgur.com/SpsBgrc.jpg

6) now go here and add this line and click save https://i.imgur.com/tufytGY.jpg

now export your PDF reports and they'll include the unit price after discount.