You can add arbitrary files to your repl, which means you can add PDFs, images, or any supplemental material. However, we recommend to start your students off with a README.md a file. It's the standard documentation and entry-point file for code repositories and when Repl.it sees that file it will display it first. Here is an example repl with a README.
You are able to create private repls on a team where only folks on the team have access. You can also create folders and store the associated lessons in each. The repls that are created can have notes from the class in markdown files or in commented out code. You are also able to upload images videos, etc. into a repl.