Contour Lines exported as raster in Illustrator

Bitmap markers are the pictorial symbols such as little fire trucks or trees sometimes used to represent points. A contour line is just a vector feature. See Exporting to PDF for tips on how map features export to Adobe software. "If a layer contains picture marker or picture fill symbology, use the option Vectorize picture markers/fills, found on the Format tab of the Options panel. This prevents rasterization of layers below picture markers and fills."