JSTOR provides access to millions of journal articles, books, images, and primary sources in 75 disciplines. The Archive Journal Collection is included.
API available, more information here: APIs for Scholarly Research
Chapter/content section downloads
Project MUSE hosts more than 800 journals and more than 90,000 books covering the humanities and social sciences from nearly 400 leading university presses, scholarly societies, and related publishers.
Full text downloads, chapter/content section downloads
PDF, Compressed PDF