Is it ethical to obtain journal articles from places other than the publisher's official sites? When I am at home and looking for journal articles I often find article PDFs  through google that have been uploaded to various websites (e.g., universities, institutions, arbitrary websites, etc...). 

I am worried that using these files may infringe copyright since these articles may be bought by these universities (for example) and (by mistake?) were freely uploaded to their domain name like: www.university.edu.lb/journal_article1.pdf (probably for their own students?).


Is it ethical to use these downloaded PDFs during my research without going back to my own university elibrary to use them?
When, if ever, would I be breaching copyright or breaching professional ethics if I accessed these files?


EDIT: my question also covers books as, sometimes, these are entirely available on some sites (whether they are institutional sites or not).
Yes, it is absolutely ethical to use these files during your research. Many publishers allow academic authors to upload a so-called "preprint" version of a paper to their own institutional websites or put them in repositories, and of course these can be used by others in research.

There's maybe just two minor things to be careful about:


You should try to verify that the preprint version is mostly up to date with the published version, comparing e.g. publication dates.
It is common practice to put the "official" publication source, i.e., the journal version, into your reference list. Nevertheless, if you can't make sure that the versions are equivalent regarding what you're citing them for, it may be necessary to state that you were using a preprint version.


In some cases, authors may put papers online even though it's technically a breach of copyright. But since you don't know the author's agreement with the publisher, you have no way of checking that, and in any case, it would be the author or institution that violates copyright, but not you.