Internet Explorer comes with an Adobe Reader add-on that is used to view PDF files in IE.
If you disable this add-on in Internet Explorer then PDFs launch in the native Adobe application and the ndOffice integration will be used (if installed).
Note, some PDF forms require the Reader add-on to be enabled so users may have to be educated how to switch back and forth.
Be aware that disabling this add-on may prevent the NetDocuments Viewer from displaying PDFs. If PDF Documents do not display in the Preview Pane.
To enable/disable the Reader add-on for IE, go to Tools > Manage Add-ons. The add-on may be under the "Run without permission" section: