In the purchase order edit screen, click on Save and send button.
After clicking on the "Save and send" button, a send window will pop up. You can proceed to,
Write an e-mail message to your vendor, and click "Send".
Download PDF.
Click "Later" to keep this PO as draft
Click "Skip" to mark this PO as "Sent".
Now you will be re-directed back to the PO list screen, where you can find the newly sent PO marked as "Sent".
You can "View/Edit", "Convert to bill", or "Send again" in its "Action" or use the filters to find other purchase orders in a certain period and/or with certain number/vendor.