This is possible by getting into the "Send Invoice" page.
Click the Invoices tab. Select the invoice you wish to send from the Invoices page.
Now, click the down arrow near the cog wheel of the invoice you selected, and then click Email Invoice. Or you can also reach this page by clicking the invoice you want, and then from the Invoice Details section, click Email Invoice.
The notification email to be sent appears on the Send Invoice page.
Click the 'Attach a file' button.
Browse your system/local folders to select the document that you want to send along with the invoice.
Note : The document(s) size should not exceed 10 MB.