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.