To add a One-off Attachment the Invoice need to first be created and saved. Once you have saved the Invoice you can edit it and add one-off attachments.
Web Portal > CRM > Invoices > Open the Invoice you would like to add the attachment to > Click on the Attachments tab > Add your Attachments > Save the Invoice. The attachments will appear on the second page of the Invoice PDF
From the App > once a new invoice is saved you can edit it to add an attachment > Attachments > Add photo from device library or take a photo.
Attachments can be image files such as (PNG, GIF and JPG) can be attached into the PDF, while other attachments (PDF, EXCEL, CSV, DOC, DOCX) will be available to select as attachments stored in Resources when emailing.