Is it ethical to include additional material in lecture notes if students buy them For most of the undergraduate courses I teach, I also prepare lecture notes and students usually buy a printed copy at a copy shop at the university. Sometimes I think it might be beneficial for the lecture notes if I include additional material which I might not cover in the course. 

Is it ethical to add additional material if most of the students will buy a printed copy and the price will go up due to the additional pages? Does it make a difference if the material is available as a pdf in the course's page in a learning management system?
It is very thoughtful of you to be concerned with the increased costs to your students; indeed, professors should be aware of and sensitive to such issues. At the same time, it is important not to go overboard with such concerns. As an educator, your primary concern should be to provide the students with the materials that you feel they need. That can legitimately include material that you are not planning to cover in the course but feel would be beneficial to at least a reasonable number of students. When weighing the benefit of this extra material against the increased cost of 1-2 euros to purchase the lecture notes, personally I feel that the educational benefit wins out against the fairly negligible amount of money involved. So yes, I would say it is ethical. 

As for distributing the extra material online for free, sure, that would work as well, but would probably send a signal to most of the students that this material is not important and should be ignored, and would dilute the pedagogical value of making the material available to the students.

Finally, note that my answer may be different if you personally were making a profit from the inclusion of the extra material. In that case there would be a clear conflict of interest that would warrant, at the very least, a good deal of extra caution and perhaps rethinking your entire approach to distributing course material.