After pulling my hair out for a while (no luck with Google searches, no Mail plug-ins on either machine, etc.), occasional macosxhints editor Kirk McElhearn suggested a solution that I hadn't even considered: rebuilding the mailbox on the MacBook Pro. Sure enough, after selecting Mailbox » Rebuild, all the troublesome PDF attachments were magically resolved -- previewing and saving both work as expected now. The odd thing about this is that, outside of messages with PDF attachments, that account was working perfectly on the MacBook Pro.
Because I came up empty when I googled for this issue before, I thought I'd post it here in case someone else is having the same problem.

