With Acrobat I can use the Crop tool to reduce the page size, but it
won't accept a negative crop to expand the page. Is there a way to do
this -- preferably using Acrobat 4 and/or free tools. Ultimately, I
need to do this to a multi-page file.

Alternatively, I start with PS and distill that. How does Distiller
set the page size -- I think the PS is a bit non-standard, the page
size I get after distilling to PDF is somewhat larger than the crop
marks, I can't work out exactly how this is determined.