As per My Understanding, You want to know if the you have unique permission on the Document Library( broken inheritance). In that case if you add a new user at Site collection, will it able to view the Documents.
Short Answer is No, as you have unique permission on that library, unless you add the new user in that library manually.
But if you want to give the Read Permission to the new User to that library then
- Create a Group, Add all User( who want to view)
- Break Inheritance, remove everybody > Add that group with read Permission and Single User as Super User.
- Now when you wana add more people to the site, simply add them in that group. they will automatically get the view permission on the library.