It's working correctly in Firefox!  I was working in another software and it suddenly did the same thing of downloading an attachment rather than previewing it. So I thought maybe it's related to Chrome even through I know I read somewhere that the problem existed in other browsers. I clicked the exact same attachment in the same invoice. Chrome and the Windows app open a download box, and Firefox opens the attached pdf. Making progress. Please update the thread if you figure out why some browsers are working but not others.

UPDATE: I tested FF one more time before posting. As I watched more closely, I realized that it is automatically downloading the file and then opening it. The files are sitting in my downloads folder. So I'm guessing QBO is supposed to do a download to a temporary folder automatically and that is no longer working (I do not want them to automatically save to my downloads file as FF is currently doing.)