An HPGL(Hewlett-Packard Graphics Language) file contains an instruction set for plotter control, developed by Hewlett-Packard. Hewlett-Packard plotters use this file to draw and print vector and raster content on the paper.
A file with PPT extension represents PowerPoint file that consists of a collection of slides for displaying as SlideShow. It specifies the Binary File Format used by Microsoft PowerPoint 97-2003. A PPT file can contain several different types of information such as text, bulleted points, images, multimedia and other embedded OLE objects. Microsoft came up with newer file format for PowerPoint, known as PPTX, from 2007 onwards that is based on Office OpenXML and is different from this binary file format.
You can also convert PLT into many other file formats. Please see the complete list below.