Plotting to PDF using any free or purchased (or built in driver in LT2007 and higher), or any other 3rd party driver to create a file is a convenience until the file name prompt stops echoing the DWG file name and presents the user with an empty box to fill.
FULLPLOTPATH is a variable added in 2006 to help overcome AutoCAD/LT's ability to overwhelm drivers that can't handle receiving the full path of the file in the plot. SImply type it at the command line in LT, and when prompted set it to 0 (zero). Your PDF driver should start placing your DWG file name in the box when prompting you for the PDF file's name.
And your troublesome Plotter or Printer driver may start behaving as well if you've had issues with them and LT.