Another trick is you can repack modern MS Office files with advzip from advancecomp. With -z4 It will use Zopfli to pack them. Further gains can be had with mor

Another trick is you can repack modern MS Office files with advzip from advancecomp. With -z4 It will use Zopfli to pack them. Further gains can be had with more rounds, (such as -i50). The resulting files can be quite a bit smaller and Office will open them without complaint. Yet another trick is you can save as binary format files. In Excel this is XLSB. Supposedly those are less compatible across Office versions however.