From the Inspection Details page, click on the invoice:

You'll see an automatically generated invoice for the inspection, like this:

Hit CMD-P or CTRL-P to print the invoice.  Then select "Print to PDF" (in Chrome, may be different in other browsers):

You'll then have a PDF of the invoice!  

P.S. Agents and clients can do this too.

New May 2018: Invoices now have a "View as PDF" link on the bottom, allowing clients and agents to directly generate the PDF of any invoice.