Means: Web Content Accessibility Guidelines 2.0
Rules that guide you in making your web content more accessible - no matter if this content is HTML, PDF or any other format for online publishing. These guidelines are worked-out by the Web Accessibility Initiative (WAI), a working group of the W3C, and are accepted worldwide.
Accessible PDFs should also conform to these guidelines.
The WCAG 2 are also an ISO standard: ISO/IEC 40500:2012.