Which currency the invoice should use
Invoice in the currency the client will wire. Your own books can show another currency later. The PDF should not.
Use the currency they will send
If the contract says USD, the invoice is in USD. If they pay euros, the invoice is in euros. Do not invoice in your home currency and ask them to convert. Their bank will, and the amount that arrives will not match the invoice.
Agree it before the work ships
Changing the currency after you have sent the PDF means a new invoice. The client has already queued the old total. Pick USD, EUR, or GBP up front, and keep the bank details for that one currency on the page.
Your local currency is not theirs
You may want the total in the currency you keep books in. That belongs on your dashboard, not on their PDF. The client should see one amount, in the currency they wire.