To see whether a payment slip was ordered to be received by mail or the PDF was downloaded by the donor himself, you have to know two parameters.
You find them if you click on a transaction and open 'individual merchant parameters':
There is two possible combinations:
|1) The donor downloaded the PDF.|
|2) The donor ordered to receive the payment slip by mail.|
You can see here how you can filter these parameters (e.g. if you want to create a list of all donors that want to receive their payment slip by mail).