How can I print a grid of PDF PCBs
In Mac OSX you can cut and paste sections from PDFs from the Preview app. I paste them into OmniGraffle and arrange by hand.

Under Linux, you could try converting the PDF to postscript (pdf2ps) then manipulating it with Inkscape. Or, you may be able to do it from a script using poster.

Alternatively, you could just save the image as a bitmap at the resolution you need, then edit the bitmap.