Is it possible make Preview not enhance thin lines? Some PDF renderers "enhance" thin lines, i.e. make them thick enough to be clearly visible, no matter how far one zooms out.  Other PDF renderers do not artificially thicken thin lines.  

Adobe Acrobat Reader allows one to choose between these two behaviors via an item in the application preferences.

The Preview application always enhances thin lines, and does not seem to provide an option to suppress this behavior.

Is there some hidden setting on the Mac which will prevent thin lines from being enhanced? (e.g. "defaults write ... ")
Preview's preferences used to contain a setting "Smooth text and line art", but it was removed from the app (around Sierra/High Sierra, I think, but can't be sure).

The plist data key associated with the preference was PVPDFAntiAliasOption. It's possible that this key will still work. You may want to try:

defaults write -app 'Preview' PVPDFAntiAliasOption 0


and then relaunch Preview. If this doesn't work, then Apple have indeed decided to permanently hold lines at any zoom level, and you'll need to use another PDF viewer for this purpose.