This error pop-up is often caused by an Adobe Acrobat settings issue:
1. Close all open PDF files, and open Acrobat by itself.
2. Go to Edit > Preferences > Security Enhanced. Click YES if you get this pop up:
3. Uncheck the “Enable Protected Mode at startup (Preview) (leave check the “Enabled Enhanced Security” but come back and uncheck it ONLY if the first step doesnt work).
4. Close Acrobat, go to NetDocuments online and open / check out a PDF file. Once opened in Acrobat, go to File and you should see the options for “Local Open” and “Local Save” (this allows you to navigate to files and folders on your computer rather than launching NetDocs). File OPEN should now launch NetDocs.