guide for employ certificate and invitation from duly authorise depart

Dear Sir,

Sir hope you will be fine.
I'm tahir Mehmood from Pakistan. I already before send inquiry to you for first time visit to China to my company.
Because I'm employ in one China based leather company and paying duties in Pakistan's leather industry.
I am not employ in Pakistan si how to I can provide any Pakistan leather tannery job prove certificate.
I already have my company employ certificate.
And also please guide me what is the letter that from China duly authorise department.

Please help me to understand for that.

Thank you.

Tahir Mehmood