If I have one PDF with different designs, can I submit that as one job?

You sure can. 

If your PDF has 3 designs and you are after 10 copies of each, that's no problem at all. But if you have 3 designs and want different quantities of each its best to place separate orders for each one. It just helps to cut out any confusion. 

This is also as long as the designs are all the same size as the size you are ordering.