If you want to apply a general discount on all jobs in the document:
There is a Discount field in every invoice/quote. That's where you can define the discount (in %) you want to grant to your client.
If you want to apply a specific discount on certain lines in your document:
There is a Discount column where you can define a specific discount for each job.
For that, you have to edit the details of the line concerned by clicking on the pencil on the right and define the discount (in %). As you change the percentage, the line total will update automatically.