Hi,
We use this code "parent.window.open(urlProvidingFile, 'blank');" then problem was solved.
SAP's answer for this problem:
It turns out the issue was caused by the fact that custom fiori apps are loaded in iframe elements, while standard fiori apps are loaded into div elements.
This means you will have to make a small change to your javascript to make it work.
All you have to do is change the lines window.open (urlProvidingFile, 'blank'); to parent.window.open(urlProvidingFile, 'blank'); and then the attachmentViewer plugin should handle opening the pdf.