According to the Docs on Esri's website for the Layout (and mapSeries) class, there is supposed to be parameter for the "exportToPDF" function called "output_as_image", but when running code including that parameter (both in IDLE and in the python window of ArcPro) arcpy gives the "Unexpected keyword argument" error.
Is this is a versioning issue where I need to update my version of Pro to the newest? I don't see anything in the changelog referencing the addition of this keyword in the exportToPDF functions.