There are two ways to create a mock invoice for your customers/potential clients.
The first and easiest way to create a mock invoice would be to just print a template. To do this, go to the Setup tab and click "Invoices" in the left menu under the "Templates" folder. From there choose the template you would like to print and click the “Click here to Preview this Template” link (seen at the bottom of the screenshot below).
The second method requires printing another customer’s invoice using the pdf link at the top of the invoice detail page. Once you have the invoice downloaded edit sensitive data using a pdf editor. Many PDF editors are available online, examples include http://www.pdfescape.com/ or http://www.nitropdf.com/.