Can I add discount on total bill?
Yes, But you have to enable an option from configuration.
Go to Setup > Configuration > Inventory Setting.
Enable a option "Enable Bill Discount in Percentage".If you want to give discount as amount you can also enable an option
"Enable Bill Discount as Fixed Amount".
Then you can see Item Discount(%) at the bottom of purchase transaction.You can enter value in percentage.
If you want to gives discount as amount you have to enter the value in "Item discount(Rs)".