Which means your browser settings are not allowing this because of security reasons. This is a pretty common problem if yo are using SSO.
Try adding you url in the Trusted Sites or in Local Intranet Site zone. Also see if the generated word document is placed in C:/Users/AppData/Local/Temp/... or in C:/Users/AppData/LowLocal/... if its in LOW that means browser/IE is preventing it.
For this to work there is no other way but to have a trade off with security, which is allowing you Pega URL to either trusted sites or in local intranet sites. Your companies policy might prevent it as well!