The program 'HpglDxf' is offered for free download on this website. For the usage a registration is required.
The version 1.0 handled pen commands with one set of target coordinates only. Numerous feeedbacks pointed out that many programs produce hpgl-files containing pen commands with several sets of target coordinates representing a complete track. HpglDxf 1.1 now supports such pen commands and also utilizes dynamical ram allocation. Thus both the operating speed and the error tolerance of the conversion could be increased.
You find 'HpglDxf' here.
The Website is here.