You must not be aware of the Advanced Settings page option to enable checkout page validations. The plugin specifically provides an option where you can enable/disable the validation of the checkout fields before the PayPal option can be used.
Here is a screenshot showing the option Validate Checkout Fields:
Enable this option if you want the PayPal plugin to validate the checkout page required fields when the PayPal button is clicked. If required fields are missing, a notice will be shown to the customer on the checkout page. PayPal has the ability to provide the billing and shipping address info automatically so we recommend leaving this option off. The checkout fields won’t be validated if your customer clicks the express checkout PayPal buttons since the fields will be populated by PayPal automatically.