Include unofficial publication in CV
I would suggest a slight variation to how you can present such work: Research Projects section in your CV. Give a short (1-sentence like) info about each research project and your contribution right inside the CV and include a link to your website, where these unpublished papers/reports are available.

Reasons:


you want to show the research experience that you had
this research is not published formally through a peer-reviewed process and you are unsure if that publication record (even if it is mere arXiV one) will serve you any good after being admitted to a Ph.D. program
you want to provide an opportunity for a potential advisor to take a closer look at the details of your previous research