This is not an issue of ArcMap or ArcGIS Pro, but is likely caused by Adobe Reader.
Go to Preferences/Page Display in Adobe Reader and switch off the option to "Enhance thin lines". This option will thicken any very narrow line work in your output PDF when displaying it in Adobe Reader, essentially ruining the display.
Unfortunately, Adobe made the ...
Highest ranked comment by @Michael Stimson on below post/question nailed it. I had a transparent layer close to top of Table of Contents. Apparently that vectorizes other layers upon output. So I removed the transparency (you can also just move it lower apparently).
Exporting inset map with same quality in ArcGIS Desktop?
Perhaps there's a better solution to this. Nonetheless, what did the thrick for me was duplicating the layers of the map containing the power plants but this time only categorising the points once (before they were categorised both by colour and size; now only colour). The layer with 2 categorisations was hidden so I could update the legend in the layout. ...
Using QGIS 3.16.x you may double-click any symbols in the legend properties dialogue and change their symbology, esp. size, which may be problematic if based on values and/or map units. This should allow you to set the symbols as you need them.