Import PDF not matching content size
 

I'm trying to open a PDF in OmniGraffle, but when it opens the document, it's showing me a US Letter portrait canvas for Tabloid landscape content, with the content centered on the smaller canvas.

When I change the canvases with Page Setup, it doesn't keep the content centered. It only extends the canvas to the right, leaving content off the page on the left.

How can I change the canvas size without having to re-center all 40 pages manually?

Also: why can't OG realize it's the wrong canvas size when it's being imported? It doesn't make sense.