Hello prabhakaran,

Before considering the solution to your requirement, you also need to consider one important perspective of scenario with tax items.
1.) There might be different Taxes configure on the products.
2.) Now, when we apply the Global Discount on PO. We also need to re-calculate the Total taxes on PO.
3.) Now, Re-calculation on taxes on a product will be easy in case of Discount given with Percentage. you just need to give a percentage discount to individual products in PO.
4.) Discount Fixed Price: It will little complex to achieve as you have to divide the discount across products in some manner that its total becomes the total amount you have given as a discount.

So Considering this scenario, you can achieve your perfect specifications.
Thanks.