You can add PayPal as a payment method for future invoices by navigating Business Settings > Payment Integrations. Once you've entered and submitted your PayPal email address, any future invoices will come with PayPal as your standard payment method by default. You can also enable/disable PayPal for individual invoices using the bottom right "Invoice Settings" button while creating or editing an invoice.
If you send out an invoice with PayPal enabled, your client will be able to click the PayPal button right on your invoice. This will redirect them to PayPal in order to make payment, and back to your invoice once the payment went through.
Please note that any invoices created before your PayPal gateway was activated do not come with PayPal support by default and will have to be adjusted manually according to the instructions above.