hpgl to pdf conversion is often used for export of large gerber schematics to PDF format, typically for the purpose of printing these physical printers. HPGL format is often supported either directly or via plugin, but it's one of the more typical export options when working with schematics.
Your best option is to use tools, such as Gerbview that can be used not only to view gerber files, but also export these to other formats, including PDF.
Updated: October 16, 2023