PRINT INVOICE
Open an existing invoice in View/Edit mode. Use the Sales Invoice options drop down list to select the Print to PDF option.

Depending on your web browser settings, you will be prompted to open or save the PDF.
EMAIL INVOICE
Open an existing invoice in View/Edit mode. Use the Sales Invoice options drop down list to select the Email option.
The fields in the Email window will be populated automatically, but can be edited as required.

Click on the Send button to transmit the email.