What are source files?

During the Programme stage, the chair of the conference can ask the authors to upload their source files. 

This is usually a file containing the same text as in your final paper (which is often a PDF) so that the organisers will have a version they can edit directly if necessary. They are usually uploaded as one of the more common document files (e.g. TXT, DOC, or DOCX).

Sometimes the chair of the conference will ask for source file in a different format, for example PDF, but this really depends of the conference organizers' decision.

If this doesn't answer your question, please contact the conference organizers for more information about the content of your source file.