To batch email invoices, proposals, work orders or other documents, first go to jobs you wish to email about (e.g. Proposals, Active Jobs, Leads).
Then, select the jobs you wish to send documents for in the list and select Message -> Email (note, you need to email recurring and one-off jobs separately):
You will then be presented with a batch email dialog. Select the email template you are going to use, review the recipients under the "Email To" section, and review the attachments. Then click "Send" to email proposals, invoices or work orders for all of the selected jobs.