The documentation of the Document Viewer says this:
The widget tries to display a document inside a page. However, it may fail to do this, since browser settings determine whether a document can be opened inside a page. In such cases, a download pop-up window is opened. For example, Internet Explorer may succeed in opening a .doc file inside a page and fail in opening a .png image, while exactly the opposite may be true for FireFox.”
You might want to check your browser settings to make sure they allow opening the document in a page.