Files with .pcx extension are marked as PCX. Its whole name is Personal Computer Exchange. It was created by ZSoft Corporation. It was used as default format for application PC Paintbrush, which was targeted on DOS as graphical editor. In its time it was very popular and it was able to use other formats, like JPG or PNG. The main advantage of this format is that it is lossless format and it was able to support up to 16 million colors.
First version vas released 29 years ago, in 1985. Six years later was released its latest stable version. That was on May, 1991.
To this day it is supported by many programs. The best known is Adobe Photoshop, which has its own trial version. It can be also opened by GIMP or IrfanView. Both of them are for free, which makes them very useful. Then there is XnView that can aldo view these files. The best program to view and edit these files is GIMP that is available for free.