Would making the fields read-only be sufficient? Without additional help, this is the only way using Reader.
If it has to be a "plain" PDF, and all your users have is Reader, you would need server support to create such a document. The tool of choice for accomplishing something like that would be FDFMerge by Appligent. Here you submit the data to the server, which then fills out a blank form and flattens the document, and then you can make the resulting document available.
Hope this can help.
Max Wyss.