Hello Faysal
Solar Panels are indeed duty free, and you are also not required to pay any advance income tax or sales tax at the time of customs clearance. You will be required to pay only the excise duty.
There are various pre-shipment documents that are required from accredited agencies, without these documents, you will not be able to clear the goods from Pakistan Customs. The documents are as following:
Invoice of shipment
Packing List
Bill of Lading
Copy of letter of credit or import contract/bank contract
Type Test Certificate
Certificate of Conformity
Pre-shipment Inspection Certificate
It is highly advisable that you go through the SRO 604 and Process Flow for Import of Solar PV Equipment in Pakistan. You can include these documents in your contract with your supplier for additional security.
Best of luck with your first shipment