I've created few reports using PSQuery and some of them have Excel as default output.
The Excel output file works well when the client PC is MS Windows and have MS Excel 2003 & 2007.
If the same report is ran on Linux and MAC, it shows excel file with XML code.
On MAC MS Office for MAC is installed and Linux uses Open Office.
MS Word & MS Excel is supported, Open Office isn't.
And MS Office is only supported on Windows, see FAQ #6
PeopleTools Certification FAQs - Additional Components [ID 756831.1]
PeopleSoft Enterprise PeopleTools Certification Table of Contents [ID 759851.1]