Letting your customers know how they can pay you is likely to be important to both you and your customer!
In the settings page scroll down to the 'Payment Methods Accepted' part of the page.
Whatever you set here is displayed on the PDF invoices generated to your customer.
If you choose 'Bank Transfer' then your bank account details are displayed at the bottom of the invoice PDF. If you select 'PayPal', a PayPal logo is also shown at the bottom of the invoice PDF.
If you choose PayPal, the PayPal logo on the PDF becomes active when viewed online. This means your customer can click the logo and be taken to a payment page that will allow them to settle your invoice using the PayPal service.
The PayPal page is automatically populated with the required amount. The receiving PayPal account is the one specified in the 'PayPal email address' setting.
Please note your customer does not need a PayPal account in order to pay using the PayPal service.
If your customer reports that the PayPal button is not working it is likely due to a setting on their computer. They will need to download the PDF and view it on a PDF viewer first.