You can provide your documents as:
HTML is what browsers use, and everyone viewing a web site has one of these, so everyone can use the content. There are resources elsewhere in this site about creating accessible HTML 5, and many other web sites and resources for advice.
This is the easiest and best option.
Note: If you have a word document, http://webaim.org/techniques/word/ tells you how to do this (WebAim has many excellent articles on converting things for web sites).
Your pdf must be Accessible, otherwise you definitely provoke the problems reported by Jacob Nielsen.
(Be advised by WebAIM: If you're working from Word, use the Adobe add-in if you can; otherwise the Word native facility)
You must provide the required accessibility features defined by WCAG, particularly 1.1 and 1.2 - where applicable.
Obviously, your intention is to provide it in that format due to the specific characteristics of the format - if you provide an image, you expect people to look at it, and you know that people who can't see pictures won't know what they're supposed to be seeing.
These are specialised or source formats. Provide them only to people you know can read them (and who will only forward them to poeple who can read them).
Source formats must only be provided if you want people who get them to be able to change them (this includes people who may receive the document because it is forwarded by someone you send it to. You must trust your recipents!)
You must provide the same accessibility features as for (3) in case somebody who can't read them bumps into them.