Automated Process user has no profile as of now. This means that when using Platform Event triggers, the Automated Process user cannot access Visualforce pages and you cannot do:
PageReference page = Page.YourVisualForcePage;
Using EventBus to create pdf attachments from a visualforce page is thus impossible.
Giving this user an actual profile, or the ability for Visualforce page access/Apex Class access will resolve this shortcoming.