Can I attach SharePoint documents to an email?

Yes, select on or more items and use the "Mail Item(s)" button in the ribbon or the option "Mail Item(s)" in the context menu of an item to generate a new email with the selected item(s) as an attachment.

This functionality is also available in Teams when the SharePoint site is shown on a separate tab. This way users can drag and drop emails from Outlook and also select online documents and attach them to a new email.