Here's an overview of which keyboard shortcuts you can use when handling documents on the Bill Pay page.
5 (after clicking a document):
Switch between the tabs on an expanded document (Details, Timeline, Payment, Integrations and Chat, respectively).
D (after clicking a pending document):
Approve or decline the expanded document.
K (after opening a document in fullscreen view):
Navigate to the previous document or to the next document.
Esc (after clicking a document or opening it in fullscreen view):
Open a document attachment in fullscreen view after clicking it, or close the fullscreen view of the document attachment.
Open and close the document search field.