I agree that the relative small size of PostgreSQL documentation and the fact that it fits in a single "book" compared to the many books that comprise the Oracle documentation can make it easier to find something. But can't say I've ever seen a product with as much quality documentation as Oracle.
You can find the documentation for Oracle 12c here: http://www.oracle.com/pls/db121/homepage
As a user, you want to look in "SQL Language Reference" first, followed by "Data Warehousing Guide" (found in a separate folder on the left side). As an administrator, "Concepts" is required reading followed by "Administrator's Guide".