In order to send an email from an Invoice, Sales Order, Purchase Order, or Quote without including a PDF, the setting first has to be enabled.

To turn this on, go to Settings > Additional Settings > Email Settings

The control for this is included with those relating to using Assistant email addresses, so you will want to tick the boxes relating to this setting for each module you need.

Once this is enabled, you can go back to your financial module(s) and simply hover over the Email PDF button to select the option to send an email without the PDF included.