Line thickness in layout & export

[Göran]

I have noticed often that in layoutview and exports from layoutview ArcGIS (currently ArcGIS Pro but problem occured in ArcMap aswell) seems to have problem with thin lines. Is there a solution to this?

My heightcurves have been set to line thickness of 0,14 and 0,4 respectively but in the output they all seem to be set to 1 pt thickness.

Dataview:

Layoutview:

[MarkMathews]

After messing with it more, if you have an aerial background.. I found that using the default JPEG2000 compression and 85% quality with overprint on and 300dpi works very well with file size and quality. 

[AlexVerville]

Thanks for the insight. I tried turning on "Simulate overprint" and even with "Output as image" unchecked, it outputs an image. That's why when you zoom in on the lines they appear grainy and also if you try to edit text in the PDF you won't be able to. Unfortunately this setting does not solve the problem but its good to know that the "Simulate overprint" option does something completely different than what ESRI says it does.