The extension PCT denotes a Macintosh PICT images file. These files are also known as QuickDraw files.
This format was developed on the Apple Macintosh and was used to record sequences of QuickDraw commands.
PCT files may include vector graphics or bitmap images. Thanks to the standard nature of PICT files as part of Quicktime, almost any graphics program on the Macintosh can open those files. There, however, usually the ending pict is used.
There are two variants of a format. The older Pict 1 could only store 8 bit graphics, while Pict 2 also has 24 or 32 bit color depth.
The PICT format has now largely been replaced by the more flexible PDF.