I was doing this:
arcpy.mapping.ExportToPDF(mxd, PDFName, resolution=600)
So I switched to this:
arcpy.mapping.ExportToPDF(mxd, PDFName, resolution=600, image_quality='NORMAL')
Right-click a layer in your TOC, and choose Edit Features: Start Editing. You get this error:
Start editing encountered one or more layers with errors or warnings and cannot continue.
No editable layers.
This can simply mean that the data frame you want to edit in is not active, an easy thing to overlook. Would be nice if the error message mentioned that.