If your invoice template is a PDF, there is a new button in the invoice detail page called Email / Print Invoice PDF (if you don’t see it, simply edit your page layout and add the button to your page). When clicked it displays a page that lets you do the following:

1. Preview the invoice

2. Email the invoice

3. Print the invoice

4. Attach PDF to the invoice record

Note that depending on your browser, the print functionality may differ from the screenshot above, but there should be an equivalent print functionality on different browsers.