How to open .sch files?
Orcad, PADS and Protel all used .sch at one point and there is no standard implied with the file type.  Orcad now uses .DSN.  Check with the vendor.  Most commercial products offer a free reader (aka viewer) or a free Lite version that limits modification but not the open, so users can view files.

Orcad makes a free viewer.  As a sidebar I also like ViewMate for Gerber viewing: