not a teacher
Of the three examples, "read when you free" jars because it is too unlike standard English, but as you are, presumably, the only person who needs to understand what the folder is, then it seems unnecessary to write a complete sentence.
On that basis, "read when you are free" strikes me as too long. I would just call it "read when free", or even something like "to be read", "read later".
Student or Learner