Destination of merged PDFs in Automator?
It is placed in a temporary file, with some random name, such as:

/var/folders/mt/k2_g0s2x3dl9jpx009p14nmw0000gm/T/JbtgKP.pdf


In general they are placed somewhere under /var/fodlers directory.

If you want to find where it is, you can open a Terminal and type something like:

sudo find /var/folders/ -name "*.pdf"


But probably, you'll want to rename and move it where you need, adding Rename Finder Items and Move Finder Items steps.