"Source files" refers to the original files or documents from which something is created or compiled. They contain the code or content that the final product or program is based on. Full definition
I, for example, do not even accept PDF files as source files for my formatting work, because the headache associated with it is tremendous. (guidohenkel.com)
If you also have the original source files, a lot of your time is saved and makes your job a lot easier. (elearningindustry.com)
Trying to do that with source files created by someone else who doesn't use styles would make us insane, so we just say no. (susandaffron.com)