How to saving a file to Media Libary by code

I'm building a plugin that works only in admin.

I've a page where the user choose some parameters and the script get data from posts and posts meta then it generate a pdf file and then I want to save it or in a specif folder or in the media library adding it to a specific Media Category.

My questions is: How can I save by code the generated pdf file to Media Library with a specific Media Category?

Regards.

Alex