Computer Configuration > Administrative Templates > Google Chrome
Both
HKLM\Software\Policies\Google\Chrome
Setting the policy to Enabled turns the internal PDF viewer off in Google Chrome, treats PDF files as a download, and lets users open PDFs with the default application. Setting the policy to Disabled means that unless users turns off the PDF plugin, it will open PDF files. If you set the policy, users can't change it in Google Chrome. If not set, users can choose whether to open PDF externally or not.