Will the user need to modify the document and then save it back to it's original location?
I don't think you do, if I understand your original post. I'd use the export Field Contents script step that this button uses, but export it to the user's temporary items folder so that the file is not retained when the user shuts down their computer.
You can also use a web viewer to view some types of documents iniside the web viewer.
If these are inserted files (not pictures) and you have used the "by reference" option to insert them. You can also open the document by double clicking the container field.
A button that uses Go to Field [select/perform] will do the same thing as the doubleclick.