Unpaywall - installation guide: http://unpaywall.org/
Unpaywall refers you to the legal, author published and authorized version of the article.
These are publications published via the green route, Open Access (behind this link you will find more information about publishing Open Access, (in dutch) ).
Open Access Button - installation guide: https://openaccessbutton.org/
When you submit a scientific article, the Open Access button searches the internet for available PDFs. If it is not found, the Open Access button starts a request to the author.
On Twitter you can use the hashtag #icanhazpdf in combination with the link of the article you can't access. Maybe someone will be able to send you the article: https://twitter.com/search?q=%23icanhazpdf.