did you ever find an answer? we have same issue.
Can you please check the following setting in your environment, anything defined/configured there?
Settings > Apps > Settings and Policies > Security Polices > Data Loss Prevention > Limit Documents to Open Only in Approved Apps
I came across the same issue. my setup was WS1 Web browser inside a WS1 Launcher not able to open pdf attachment from web mail.
Resolution:
Settings -- > Apps-- > Settings and Policies --> Security Polices-- > under Data Loss Prevention Section ---> Limit Documents to Open Only in Approved Apps list Added pdf reader App.