Is Sharepoint's read-only mode really read-only?
When you open a document a document, you actually download it to your computer and open it in the appropriate application. What this app lets you do then is not related to SharePoint.
What you can do is set up permissions in the document library so that only some people can add/edit documents and others can only read. This way, the latter will be able to open the documents, do whatever they want with it and not write the modified version back.