You can reprint Purchase Orders via the Order Documents function as follows:
Purchasing
Transactions
Order Documents
Select the required output destination
Ensure the "Print the following" option is set to "All" and "Purchase orders" is ticked
If the Purchase Order was in a currency, then select the Home and Currency option
Then click "Simple Search" (the funnel symbol located in the middle of the tab at the top of the window)
Enter search criteria as required
Ensure the Period Range value is set appropriately for the order you are wanting to print
Click Print
You can also view completed/closed Purchase Orders by selecting (but this is a transaction list rather than an individual formatted document) …
Home
Purchase Transactions
Outstanding Purchase Orders
Tick List Definition
Tick the box for Completed
Refine the search using the filters and periods range at the top of the window
Click Display