All web documents should be divided into short sections for readability, and should be organized using a clear hierarchy of headings and subheadings. This is true regardless of whether the document is delivered using HTML, Microsoft® Word, Adobe® PDF, or another document format.
In all formats, it's important that headings be explicitly defined as headings, rather than simply formatted as larger or bolder, in which case they may look like headings but they aren't actually headings. In HTML, headings should be defined using valid HTML heading elements such as H1, H2, and H3. In Microsoft Word, headings should be defined by selecting appropriate heading styles from the Formatting Toolbar. When a PDF is created from Microsoft Word, these heading styles can be passed on to the PDF as bookmarks, which facilitates navigation within the PDF document.
Using a clear, explicitly-defined heading structure helps all users to understand the document contents, but it especially helps users of assistive technologies such as screen readers. Most screen readers include functionality that allows users to jump between headings with a single keystroke, so blind users can effectively scan the document looking for sections that particularly interest them, just as sighted users typically do.
Another technique for facilitating navigation within a web page is to include a same-page link that allows users to jump from the top of a document to a particular anchor within the document. This technique is most commonly used for allowing users to skip past a redundant navigational menu and go straight to the main contents of the page. For additional information about this technique, consult the AccessIT Knowledge Base articles What is a "skip navigation" link? and Is it a good idea to make "skip navigation" links invisible?