They're open in parallel.
nautilus will spawn as many processes (the order doesn't really matter here) and each of those processes will execute your PDF viewer with one of the files as arguments.
All those PDF viewers will work independently, load shared libraries, open their own PDF files, connect to the X server and at some point ask the X server to display a Window. Which one will do that first you have no control on.
To guarantee the order, you'd need cooperation from the Windows manager. You'd need nautilus to ask the Window manager to tell it when the window for the first PDF viewer has popped up before running the next one.
Doing that reliably for any application is not easy.
nautilus only knows the pid of the process it spawned to execute the PDF viewer. Not all applications tell the Window manager their pid, and if they do, it may very well be the pid of a child of the process spawned by
nautilus (like in the case of a shell script), or it may very well not create a window at all...