You cannot edit a document in a web form. First, only a copy of the document
is downloaded by the browser. Second, the browser cannot open these sorts of
documents. It uses external applications (such as Word, Excel, and Adobe
Reader) to open them. Now, let's say that a user downloads a Word document
and edits it. He/she is editing a *copy* of a document on the server. The
user therefore has to have a method of uploading the new version of the
document and saving it over the old. This is no easy task either. First, as
the document is not loaded in a browser, the user cannot save it using a
browser button click. The application that the browser uses to open the
document would have to do it. To do this, the app would have to know the URL
of the original document, be able to upload the document to the server, and
the server would have to be able to store the document in the appropriate
location.

We haven't even discussed security yet. You don't want everybody and their
brother to be able to upload and save documents over existing documents in a
web site. you may even want to keep backup copies of different versions on
the server. You may want users to be able to collaborate in some fashion.
All of this requires a good security model, and a well-thought-out desing of
both the client and server software to accomplish.

In other words, if you're an expert programmer, with a great deal of
knowledge about ASP.Net, HTTP, Security, and the client applications that
would be used for editing the documents, and if you had a team of a
half-dozen expert programmers to help you, you might be able to architect
and build such an app in 6 months to a year.

Your best bet is to use something like Windows Sharepoint Services, which
has all of this functionality built in.