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.