Chemistry Europe is an association of 16 chemical societies from 15 European countries. It publishes a family of high-quality scholarly chemistry journals, covering a very broad range of disciplines. After you search, look for the "Open Access Content" limiter on the left side of the screen.
This website allows researchers to upload their science articles before they're published in an official journal. The articles may not yet be peer-reviewed and will likely undergo revisions, but they can be a good source of information.
CORE is the world’s largest aggregator of open access research papers from repositories and journals. Many full-text PDFs are available online, but you can also request articles through Interlibrary Loan.
"A pioneer of open access publishing, BMC has an evolving portfolio of high quality peer-reviewed journals." Most articles have a full-text PDF available.