Set PDF-XChange Editor as the Default PDF Application
It is important to note that while per-machine options for setting default file associations are available to installers during deployment, they often cannot be applied to existing user preferences. Users normally have the necessary permissions to disable PDF-XChange Editor from being the default application for PDF (and other) files after the application has been installed. Similarly, other applications may set themselves as the default application for specific file formats when they update, or users may select an alternative application as the default application.
Window 10 vs Earlier Versions of Windows
Setting file associations programatically was relatively simple in earlier versions of Windows. Microsoft made this process more difficult from Windows 10 as a security measure against malicious software making unwanted changes to user systems. Setting registry keys is no longer sufficient as a result of these security measures, and user interaction is now necessary in order to set file associations.
User Sessions and the Windows Registry
Although a system-wide (per-machine) preference can be set in HKLM, users may also have their own file associations specified in HKCU. If an installation of PDF-XChange Editor take place and the default .pdf associations are specified for PDF-XChange Editor in the HKLM, then all users without existing .pdf file associations will have PDF-XChange Editor set as their default application for .pdf files on the first run. However, this association will not affect the settings for users who already have an existing .pdf file association on their machine, and PDF-XChange Editor will not be set as the default application for those users.
This problem can be mitigated by removal of the key:
Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pdf
User preferences for .pdf file associations will revert to the system-wide (per-machine) setting when this key has been removed.