How to execute a task after a determined time?

Hello, how to execute a task after a determined time?

Here is my problem, I generate PDFs using an API, but the result returned to me is the PDF itself, so I cannot tell it at the end of the API execution, launch me a another script because it does not receive the correct response from the API.

I therefore wish that after 20 seconds my page triggers a server connect script

20 seconds is an example, this is the average execution time of my API