SUMMARY

PDF content with thin lines (like engineering drawings) are not shown as thin lines in the viewer, but rather all line weights seem to be rendered at the same thickness.

there is a backend setting for Enhance thin lines: but setting this to "no" or any of the other settings seems to have no effect.


STEPS TO REPRODUCE
1. open a PDF created with different line weights, including thin lines
2. 
3. 

OBSERVED RESULT
lines appear to be all the same weight (heavy).

EXPECTED RESULT
line weights are rendered correctly or are at least adjustable via the backend settings.

relevant discussion about this topic here
https://discuss.kde.org/t/okular-line-weight/43636