If you have the AutoPagex plugin
http://www.evermap.com/autopage.asp (see pdf size thread -- demo free but with DEMO watermark) you can in acrobat
1. Use AutoPagex DividePages to split a pdf page to 2 halves (upper/lower)
2. Crop to remove all margins
3. Rotate 90 degrees conter-clockwise
and the result is not too bad, although your pdf file will be quite big. You will have one half pdf page to each iRex page.
Not perfect, and a bit laborious, but better than original unreadable format.
Sorry about my photography skills, here's the before and after: