Looking for a manual

Hello.
I am trying to build my first project using pixhawk.
What I would like to do is manipulate the PWM output by the USB cable connected to a PC (write a program)
Is that possible ?
Is there a relevant manual I can look at ?

Thank you

Take a look at this page, it has a lot of links for the entire documentation of the project

https://www.dronecode.org/documentation/

thank you for the link