Now invoices for the transactions you have had with Site24x7 can be obtained in the Accounts -> Billing section of the Site24x7 client.
Invoices of last 5 months also can be obtained in the above section. Going forward you will be able to get invoices for all the transactions done with Site24x7.
To view the invoice, go to Accounts Tab -> Billing section. Under Billing Details and Invoices, you can see month-wise billing details. Along with billing details, you can find the links to appropriate Invoices.
Click on the 'Invoice' link, this will open the corresponding invoice in a PDF.