Accessing full text articles is a bit of an arduous process for a systematic review. Doing these 3-4 steps in order is usually most efficient.
One useful feature that Paperpile offers is the ability to use your library's proxy. This feature can help you gain access to articles and papers that might not be publicly available. To use this feature, simply go to your Paperpile settings and input your library's proxy information. Once you've done that, you'll be able to bulk import PDFs using your library login.
Google and Google Scholar will help you find pre-print full-texts of papers that aren’t easily accessed via Paperpile.
If Google and Google Scholar fail, Sci-Hub is a website that provides free access to millions of scientific articles, but is being sued for illegally breaching copyright. Formally speaking, you shouldn’t use this… … but some people do.
If you can't find the full text of an article through a database or Google Scholar, you can request it through interlibrary loan. Interlibrary loan allows you to borrow materials from other libraries. You will need to provide the title and author of the article, as well as your name and contact information.
Rough, un-edited demo of this process