I believe you will need quotes around the path portion as well as the whole command string; for example:
STRPCCMD ‘start ”’k:\my documents\monthend.xls”” (I think you need three apostrophes to make it work as a single ‘ in CL/RPG).
Or, you could use the 8.3 abbreviation for the folder:
STRPCCMD ‘start k:\mydocu~1\monthend.xls’. (Using the 8.3 abbreviation for part of your path will not force you to use 8.3 naming for the whole thing, so there is no reason to abbreviate ‘monthend.xls’, even though it is longer than 8.3.)