PayPal Pro allows you to process all major credit cards directly on your website the same you would with any other credit card processor (ie. Stripe, Square, Intuit, etc.)
Canada and American Express
If you are using PayPal Pro with an account based in Canada, you will not be able to process American Express credit cards with the USD currency…without a bit of a work-around.
PayPal simply does not support this directly with PayPal Pro. However, the PayFlow gateway (which is owned by PayPal) does allow you to “bring your own processor”. This method does provide a solution to this problem.
Solving the Problem
In order to get around this issue, what you can do is create a merchant account with American Express directly. Once you have this account created, you can then get a PayPal PayFlow account setup, and hook your American Express merchant account up to this.
Effectively, PayPal is no longer processing your American Express payments. Instead, American Express is doing that directly, which eliminates the problem.