If you wants to print your invoice so you can follow given steps:
Step 1:You need to go to the transaction menu,open that bill in modify mode,
Step 2:Here you can see the Print option in the title bar,click on it and print in standard format or as you want.
If you want to print option after saving voucher,you can follow given steps:
Step 1:Go to the Administration menu,
Step 2:Click on Configuration,
Step 3:Click voucher series configuration,
Step 4:Click on sales and select series,
Step 5:Here you can find an option is Print sales invoice after saving,specify Y,then save this window,
Now,when you enter a voucher and after saving you will get a prompt to print sale invoice.