Then I came across another link that suggested I use pdftk (pdf toolkit). It's described here, but I got it using my new MacPorts install.
The usage is
It worked great. The only problem is the files were named: Scan1.pdf, Scan2.pdf, etc. and when sorted the order is Scan1.pdf, Scan10.pdf, etc., so they were out of order in the merge. I adopted a embarassingly inelegant solution. I shortened the names and then entered this command:
Looks great!