From the site:
“Nonprofit Explorer includes nonprofit tax return data released by the IRS in April 2013 for tax returns processed by the IRS during 2012. This generally consists of filings for the 2011 fiscal year, but may include older records. The IRS data release includes only a subset of what can be found in the full Form 990s. Wherever possible, we’ve linked to PDFs of the full 990 for each organization, some going back ten years, collected at public.resource.org.”
ProPublica also has an excellent guide for investigating nonprofits.