All was well with exporting my print files to pdf format until... I upgraded to ubuntu 19. I thought the problem would pass after I upgraded to ubuntu 20... no dice. Still *almost* invisible lines from the drawings (architectural).
Taking the dxf file from my ubuntu machine and loading on LibreCAD on my Windows 10 machine and rendering a pdf there works without issue.
Please don't make me downgrade back to ubuntu 18... please!
There has been a case recently here in the forum where a user complained when exporting to pdf all line thicknesses are rendered the same. It turned out to be an issue with his pdf-viewer, a setting for "show line thickness" was switched to off. May be something related in your case.
Still it's queer you get very thin lines. Maybe you indeed used thin lines (line width 0), and your original pdf viewer did it right. How does it look in Print Preview and on a real print out? More to examine.
That was it! Seems the older ubuntu pdf viewer/printer ignored line width. The new one is maliciously compliant. My default line widths were set to thin. I have changed it to .05mm line width as a default and now my viewer, printer and plotter (and clients) are happy.