In the Sales app, create one Product which corresponds to your subscription item.
Then create a first Invoice with the product and any linked items (company, contact, etc…). Use the Product Description/Note field within the Invoice Product list to enter the Billing Period.
Note that there is also a note field for the Product in the Products section, which is copied over to the Invoice when you first add the product, but you can then edit the field in the Invoice independantly.
For the next period, simply Duplicate the first invoice (function available in the web app only), change the invoice number and product description to the new billing period.
Invoices can be exported as PDF, both from your web account and from your phone.