It ain't pretty but it can be done.
Two step process comes to mind.
Print the document to Adobe PDF printer. There are drawbacks to refrying a pdf in this way. Color management can be lost, hyperlinks, bookmarks...everything not intended for paper. You would need to define a custom paper size smaller than letter, Adobe PDF Printer does not have a non-printing area. Toggle Scaling - Fit to Page
(or you could choose another printer most of which would have unprintable area - choose Fit To Page in the Scaling. Toggle Print to File. This will create a .ps file that you would open/drag-into Distiller)
From that reduced document use the Page Panel to define the document as Letter. Pages > Crop Pages > Change Page Sizes (toggle All) Letter