The joy of the web is that you can deliver all sorts of different ‘media’ via a single interface. Tim Berners-Lee designed it with exactly that in mind.
That is good news in terms of accessibility, because it means you can offer the same content in a choice of formats, PDF being one example.
The Adobe PDF reader has many accessibility features:
When creating PDFs always use the most up-to-date version and use the new features developed by Adobe, to ensure your documents are accessible to your widest possible audience.
Unfortunately, despite the recent improvements in the accessibility features, it is still quite difficult to create fully accessible PDF documents. For the best results, use all the techniques available (and features built in to the latest version) and employ suitable users to test your documents.
Here are a few resources that can help:
Note: HTML is a more accessible format than PDF; I am not suggesting you use PDF as your default way of delivering documents on the web.