A web document is similar in concept to a web page, but also satisfies the following broader definition by W3C:
"... Every Web document has its own URI. Note that a Web document is not the same as a file: a single Web document can be available in many different formats and languages, and a single file, for example a PHP script, may be responsible for generating a large number of Web documents with different URIs. A Web document is defined as something that has a URI and can return representations (responses in a format such as HTML or JPEG or RDF) of the identified resource in response to HTTP requests. In technical literature ... the term Information Resource is used instead of Web document.".
The term "web document" has been used as a fuzzy term in many sources but in all of them the W3C definition given above applies. Recent research in fields like "Web Document Retrieval" and "Web Document Analysis" has revived interest in clarifying the correct use of the term.
The key idea is that a single underlying resource in an HTTP system, may have several different representations, which can be exposed by mechanisms such as content negotiation.