OH MY GOSH! I'm surprised there are not more people complaining about this. 
So far, I have only discovered a few other threads regarding this issue.

This QUICKBOOKS ONLINE issue..IS SO FRUSTRATING

(is this also happening in the desktop version??)

QUICKBOOKS ENGINEERS

What setting did you turn on/off that is preventing users from being able to view their PDF file attachments in a new tab or popup window????

I do not want to change all my computer doc defaults to a different file type from PDF just because QBO isn't supporting it anymore. 

Well, it appears that they have fixed it again. I just posted the following in reply to the announcement by a QB Team member in another thread:

Thank you, Mich_S. I tried an attachment just now and can confirm that it is indeed working correctly again.

Is it going to stay fixed this time? As you have alluded to, any files that have been attached during the "bug" period will have to be deleted and re-uploaded if we want them to exhibit proper viewing behavior. I have not been attaching any files but instead saving them up in the hope that this would be fixed again. Now it will take me half a day to upload all of those. I shudder to think how much work this will be for more serious users who have dozens or even hundreds of transactions a day.

Hello there, @Roland12.

This post appears to be a duplicate of the one to which my colleague has replied. You can check this direct link to view her response and suggestions: https://quickbooks.intuit.com/learn-support/en-us/other-questions/re-why-are-bill-purchase-order-inv....

For future reference, feel free to visit the QuickBooks Online Help page. It contains helpful articles and video tutorials depending on your concern.

Keep in touch so we can assist you further with attachments or QuickBooks. Have a good one!