Can I submit a non-research "article" on ArXiv? Can a generic article be published on arXiv? For example, can I publish an article on "Introduction to Linear Algebra," or does it have to be an innovative research paper or preprint?

If not, are there any journals (e.g., open access) that accept "articles" as papers?
See Is it possible to upload expository papers to arXiv? .

I have seen linear algebra notes on the arXiv, and also notes on other undergraduate-ish subjects (including some pretty good ones, e.g., http://arxiv.org/abs/1302.6946 and http://arxiv.org/abs/0805.0585 ). I absolutely don't mind them, and I don't think others do (although I would suggest using github for course notes due to the possibility of frequent corrections and updates). Expository papers for nonstandard material definitely fit well on the arXiv, and there are several; e.g., almost every chapter of the Tamari Memorial Festschrift has been posted on the arXiv.