GPO : open PDF externally for a list of URL

Ben42 Hello!  Just to confirm, basically you are looking for the 

AlwaysOpenPdfExternally (https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#alwaysopenpdfexternally) to be limited to a list of URLs?  

I'm not sure if this will work in your specific scenario but just an idea.  Have you tried to configure the File Type Association of Win10 OS to open your chosen PDF reader for PDF Files and then utilize AutoOpenAllowedForURLs and AutoOpenFileTypes (https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#autoopenallowedforurls and https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#autoopenfiletypes) to automatically open PDF files on download?

-Kelly