You add a parameter to the report to retrieve information for the customer selected in your parameter, and then you run the report for each customer programmatically, depending on your needs there are various ways to do it. If you have a fixed number of only a few customers, you could schedule a call in a shell script to open report designer to run the report for each customer passing the parameter and the output you want.
You could also mount a Pentaho Server, publish your report in the server and use the server Schedule ability to do the same (program one schedule per customer and send the pdf output in a email per customer, or keep the output in the server and then download the output from the server)
If the number of customers is variable or the number is high, you'll need to somehow programatically call the report for each customer, you could do this with Pentaho Data Integration, it has a step to call a report created with Pentaho Report Designer, and you could use previous steps to get the customers to call the report and send the output by mail, to a specific path in your server/PC, or whatever you need. Pentaho Data Integration can run in your PC or you can mount a server as well, or use Pentaho Server.
You could use also other tools to do the same instead of Pentaho Server or Pentaho Data Integration, depending on what you already have in your company/PC, you can program calls to Pentaho Report Designer, those are complementary tools in the Pentaho Suite, but you might be familiar with other tools to achieve the same result, basically, what you need is a programatic way to call your report passing different parameters (customer) and outputs (the pdf name you want to get).
Regards