Go to your Purchase Orders page.

Choose the PO you want to share with your supplier.

You should click the "Share" button on the right of the page.

After clicking the Share button, you can see on the following screen the subject, message, .pdf, and .csv documents are filled by default according to the PO you have selected. You can customize your message if you want. After choosing the supplier you want to share, you're done! Just click the share button.

If you don't know how to create a PO, check the following article about the details