Cannot access wp-invoice help doc

Dear Sir/Madam,

I read through the Plugins support page for WP-Invoice – Web Invoice and Billing, I found every document links won’t be redirected back to https://www.usabilitydynamics.com/

Here are some links I visited but none of them can be accessed properly.
https://www.usabilitydynamics.com/product/wp-invoice/docs/home

https://www.usabilitydynamics.com/product/wp-invoice/docs/wp-invoice-plugin-just-shortcode-on-the-page

https://www.usabilitydynamics.com/product/wp-invoice/docs/empty-blank-invoice-page-or-just-wp-invoice-instead-of-invoice-in-wp-invoice-plugin

The page I need help with: [log in to see the link]