Do my clients need a PayPal account to pay through PayPal?

Typically, your clients do not need a PayPal account to pay their Invoices through PayPal. There is an exception to this rule however:

Within PayPal Standard, there is an option for users based in the US called PayPal Business Payments. PayPal Business Payments enables you to collect online payments for a flat fee instead of a percentage - $0.50 per transaction. There is an Invoice limit of $10,000, and clients will need to pay from their own PayPal account. Your client is not able to use a credit card if you use PayPal Business Payments.