If you always want to save exported PDFs to the same location, the easiest way is with Default Folder X, as it lets you, you guessed it, set default folders by app. You can see an example here:
However, if you don't already have Default Folder, and don't know if it's worth the money, you can also automate changing the save location by having KM type the ⇧⌘G keystroke when the save dialog opens, then insert the path to your preferred destination:
Export PDF Save Dialog
Go to the Folder Window Triggered by ⇧⌘G
New Path Inserted
If you look around the forum, there have been examples of this exact kind of automation shown before, so if you run into any issues getting this to work on your own you should be able to find something that worked for similar situations for someone else.