probably defaulting to 97 dpi, try to set the output parameters with print to pdf vs. export/save to pdf.
It's default is 150 DPI. Would that make a difference?
I'd ensure that 'Output as Image' and 'Compress vector graphics' are unchecked.
Adjust/increase the DPI settings also as Richard says. Unsure of the size of your layout, but a larger layout (page size) will generally require a larger DPI. 150 seems on the low side for something you would want to look nice.
I just looked at the upload I made and it's completely clear. Must be my adobe.
just and FYI, adobe remembers your last settings. So if you use a Adobe driver 'outside of ArcGIS Pro' for printing those settings may carry over into ArcGIS Pro (or ArcMap).