To add default description for a product you can check the steps from the link below.
Step 1: To add description to the product while creating the invoice please Add the product.
Step 2: Enable show description option.
Step 3: Click on add description button below the product name >> and add the product description and Click on save.
You can also highlight the description by making it bold / Italic / underline etc.
Note: The description of product what you are adding while creating the invoice will be shown for that particular invoice only.