There are two browser extensions that you can use to find free legal PDFs of journal articles. These tools look through places where authors have uploaded their work for free reading and give you access to it.
1. Kopernio
When you visit an article page on any academic website, Kopernio searches for full text PDFs and gives you one-click access via a button that is in a consistent, convenient place.
Access Kopernio here: https://kopernio.com/
2. Unpaywall
An open database of 27,095,546 free scholarly articles. Unpaywall harvests Open Access content from over 50,000 publishers and repositories, and makes it easy to find, track, and use.
Access it here: https://unpaywall.org/