Downloading Books and Research articles
There are several websites and apps where you can download free research articles, books, and multimedia files
Library Genesis : It is a database of over 2 million (yes, million) papers, articles, entire journals, and non-fiction books. They also have comics, fiction books, and books in many non-english languages.
Sci-Hub : You can basically download 99% of all scientific articles and papers on SciHub. Just enter the DOI to download the papers you need for free from scihub.
Try this Sci-Hub link if the previous one is not working
4shared: You can also get many books and multimedia files from this site
CSULB California State University : Provides unlimited free access to thousands of electronic books
PDF Drive: It is a search engine for PDF files. As of today, there are more than 73,627,882 eBooks for download for free. No annoying ads, no download limits
Researcher: It is a very handy app that can be downloaded from google play. If you like browsing through a research article at your fingertip, it is a perfect solution.
Project Gutenberg : It offers over 57,000 free eBooks