This is a post which made me share my views with all people as in used to ask many people how to copy a Purchase Order, but I used to get wierd answers saying this or that but never was able to reach to current Solution with other people's guidance.
Lastly i thought of crafting what I figured out.
Basically there are 3 Methods of achieving it i.e. of copying a PO with reference.
1. Using your left pane i.e. Document Overview

Here in the left pane is visible which appears below Document Overview off Button on Left Side of the image, If the left pane does not appear just click on Document Overview button to make it visible, basically it is hidden in most of the cases.
Selection screen will display. give necessary parameters and list of the relevant purchase orders will come up.
Single click or select the required PO and click on Adopt button, the button that appears beside the Selection Variant button appears like to square boxes
or Double click on required PO.2. Using the Cart Button
Follow the procedure till step 3 of method 1 and then after click on PO Number
then drag and drop it in the CART (at left or doc type), the cart symbol that appears above.3. Using the Line Item Method

All the methods require the click on save button as in it will create a copy then you just need to save it.
Note: Sometimes some of the line item data i.e. import data does not get copied, so you need to do it manually.