The polygon outline width of less than 1 pt is increased to 1 pt thickness in an exported PDF file when using the 'Export Map to File' function on the Share tab in the ArcGIS Pro ribbon.
In Adobe Reader or Acrobat, the settings on Preferences > Page Display has the Enchance Thin Lines option checked, which thickens thin lines. The line is 0.4 pts as confirmed in Adobe Illustrator.