When you try to open .pps file it instantly goes to presentation mode without allowing you to make any editing. Even if you try to load that document into PowerPoint program, it still runs in presentation mode.
This can be easily bypassed by renaming the extension from .pps to .ppt. Now PowerPoint thinks that it is simple PPT presentation which you coul edit like you want.
I don’t know if .pps is different from .ppt file format only in the extension but it works.