Hello there 6greenearth,
I understand you're looking for assistance with PDF documents while using iCloud Mail on your windows computer, right? Check out the following information to see if it's helpful.
With iCloud Mail on your windows computer, you would want to set this up via Microsoft Outlook to view your mail. To confirm that this is set up the same way, check out the section labeled "Set up iCloud for Mail on your Windows computer" of the following resource:
Set up iCloud for Mail on all your devices
If you see that you have iCloud Mail set up the same way, are you seeing that the PDF was created via Adobe, or was it set up with an Apple program that converts to a PDF? To test this, are you able to send the PDF to both email accounts that you have to see if it opens the same on both accounts? Also, check to see if you may need to allow access to Adobe Reader for iCloud Mail to see if that may be what is preventing a preview before saving the document.
If that isn't helpful with what you're seeing occur, Contact Apple Support so this can be looked into in more detail with you.
Have a great day!