Just to inform, the PLT files are created with printer drivers. Most common looks like to be HPGL. So you print the drawing into file using PLT-plotter.
Looks like Draftsight (free from Dassault) will save PLT files (its listed as a feature) although I don't use it myself. Inkscape (free) may also work, it does export PLT. You could export a DXF from SolidWorks to either application.
Note, PLT format is all line segments, including curves.
If you need arcs in the curved areas, you would need a different solution. There are some utilities that will save HPGL2, although I don't know of any that are free.