Complex sentence: ADS now has agreements with almost all astronomical journals, who supply abstracts, and scanned articles from as far back as the early 19th century are available via the service, which now contains over five million documents.
Split into simpler sentences: ADS now has agreements with almost all astronomical journals, who supply abstracts. Scanned articles from as far back as the early 19th century are available via the service, which now contains over five million documents.