I created a tiff shadow of my area of interest in the hopes that I could contain my complete project within ArcGIS Pro and not have to do any post-editing in Adobe Illustrator or Photoshop. Here it is in a map window in ArcGIS Pro:
When I put it in a layout and export the layout to PDF, I get this:
Interestingly, when I change my map frame a little bit but stay at the same scale, I get different results:
This one is not nearly as bad, but it still has some extraneous horizontal lines.
Anybody have a guess what's going on here or how to fix it? The same thing happened when exporting to .aix, which makes sense to me because I think it's basically the same export process. My current workaround is exporting everything but the shadow as an .aix, exporting the shadow separately as a png, and inserting it in the appropriate place in Illustrator. Obviously that's not ideal (and defeats the purpose of having made the shadow a geotiff in the first place...).
I'm not sure if it's something to do with my machine or Pro, and I can't currently test anywhere else. I'm running ArcGIS Pro 2.5.1 on Windows 10 Enterprise 1903 with 64 GB of RAM. I'm attaching the shadow geotiff (tiff and world file).