When opening a PDF File from harmon.ie plug-in in Outlook (lying on SharePoint) the xchange PDF Client opens but not the file I double-clicked. With Adobe PDF Reader it works and the file opens, but not with PDF XChange.
It means your PDF reader does not work WebDAV. Register the below registry key then check again. Copy this in a .reg file and register it
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Mainsoft\Prefs\AcrobatSPI] @="false"
---- Jean