Thanks. It is unchecked.
In fact, most, if not all, of my pdf are displayed in Windows Evernote pdf viewer. Only those added directly by Mobile app.
I have some new observations. If I do the Share/Add from the mobile app to Evernote, the pdf will not display in Windows Evernote pdf viewer. If I do a save as pdf from the mobile app, then use Evernote to add the pdf. It will display in Windows Evernote pdf viewer.
For now, I just have to do the additional step of saving as pdf onto my iPhone storage first, before manually use Evernote to add the saved pdf.