The FileMaker web viewer can be controlled via the script step Set Web Viewer. This script step is dependent on you naming the web viewer object and telling the script step that name. I have often found that I like to make the entire web viewer a button for things like downloading a document from SuperContainer. The absolute key to this working is that your must first name the web viewer BEFORE making it a button. If you do so afterword, only the button has an object name and not the web viewer. Since the web viewer would not have an object name in this case, it would not respond to the script step.
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