I'm wondering whether you can try to build from source:
http://wiki.librecad.org/index.php/LibreCAD_Installation_from_Source#Building_LibreCAD-2.0_on_OS.2FXPrint -> Save As PDF, works here for me with line thickness.
pepekpepek wrote
Hello All, I am extremely happy using this software, however problem as described happened. I tried to install on other MAC and same situation, everything worked until I printed file to PDF. Then I can't see line thickness any more and can't print or see print preview. Can somebody help me?