The most simple tool: use pdftk (or pdftk.exe, if you are on Windows):
pdftk 10_MB.pdf 100_MB.pdf cat output 110_MB.pdf
This will be a valid PDF. Download pdftk here.
Update: if you want really large (and valid!), non-optimized PDFs, use this command:
pdftk 100MB.pdf 100MB.pdf 100MB.pdf 100MB.pdf 100MB.pdf cat output 500_MB.pdf
or even (if you are on Linux, Unix or Mac OS X):
pdftk $(for i in $(seq 1 100); do echo -n "100MB.pdf "; done) cat output 10_GB.pdf