You can get Custom number for orders, invoices, delivery slips and credit slips module here: http://addons.prestashop.com/en/billing-invoicing-prestashop-modules/20202-custom-number-ordersinvoicesdelivery-and-credit-slips.html
For more info, you can contact us here: http://singleton-software.com
This module change the reference numbers from old format like #IN000001 or #DE000002 to something meaningful, what you can customize as you want. Makes it easier to find pdf documents for you and also for your customers. Customize easily your order numbers, invoice numbers delivery slip numbers and credit slip numbers as you wish for example to INV-0100/15 or SLIP-0150/15.
This Module supports multistore. You can create your own reference numbers format for each shop separately.
The custom reference numbers module supports counter option, which you can use to generate iterative values for each next custom reference number. For counter you can set starting value and a step value and you can reset this counter every time you want. You can set automatic counter reset to every week or every month or every year or counter reset when a value or a selected date is reached. Also you can set counter length from 2 to 9, for example when counter value will be 68 and counter length will be set to 4, final counter value will be 0068 (prefixed by zeros). At configuration of reference number you can always see actual next counter number for next reference number.