sitx is an archive, you have to open it or ask for the fonts again.
Sitx and the older Sit are compressed archive files created by Stuffit.
You can download a free converter for Mac at: Open RAR & ZIP Files on Mac - Free File Extractor StuffIt Expander Mac
Check the system requirements to be sure you can run the software.
Once you have unstuffed the sitx, the font files should be added to your font folders just like all your other fonts.
They’re likely Mac format fonts and there’s no way to work with them on Windows.