Assuming that pdf is returned in a body of the response, the difference between Time To Last Byte and Time To First Byte (https://support.smartbear.com/loadcomplete/docs/testing-with/results/analyzing.html#response-time) will give you the file download time.
Can you describe what for do you need to have a response to be stored as a file on the disk?
> We have a requirement to load test payslip download time
You will get this time as a Time To Last Byte metric.
-- Is your tested application a subject of in-house use or can be used by anyone from the internet? If the latter, how are you going to cope with the delays provided by the infrastructure? (Network, routers, etc.)
And still I don't see the the reason to store downloaded file to disk. The goal of your load test is to verify that your server can serve certain amount of concurrent users/requests.
Verification that the given payslip contains correct expected data, generally speaking, must be done with the help of the regular functional tests.
So the question about the necessity to store obtained response to hard drive still remains...