Bummer that you don’t have the dwg files. It sounds like you know how to convert from plt to pdf: Open in CAD, Plot to PDF.
http://forums.autodesk.com/t5/Printing-and-Plotting/Converting-PLT-to-PDF/m-p/1407309
http://rkmcswain.blogspot.com/2005/09/convert-plt-to-dwgdxf.html
I think what you’re really asking is how to get better resolution from the plt files, which might not be possible since the plt file is a means of communicating with the plotter and lacks data from the original dwg. You might want to try converting the plt back to dwg and then plotting to pdf. You’ll end up with poly-lines but they should plot better for you.
http://www.cad-2.com/plt2dwg.htm