You will first need to know where the grandfather lived to find out where to look. There is the "Kriegsarchiv" (War Archive) of the Austrian State Archives which claims to hold
"Belohnungsakten" (reward files), which included badges of courage etc. ("old" and "new" files)
It also says that
[t]he Kriegsarchiv does not keep all documents on military personnel before 1918. As from 1868, records were decentralised so that the documents remained in the provinces and successor states when the dual monarchy broke down
That means that depending on where in the monarchy the grandfather lived something might be in Vienna, but could also be in Budapest, Praha, Trnava, Warszawa etc.