PDI is a Pre-Delivery Inspection. It is a list of items to verify your rig has everything you expected and that it WORKS before you pick it up and take it home. Don't pick it up until it passes everything!
Go here first:
http://www.montanaowners.com/RVRelatedLinks.html then click on the PDI link and you'll find several versions. They contain checklists to ensure you check that everything works on the rig and you understand where everything is located, how it works and becoming familiar with any subleties that you might not have been aware of otherwise.
For example, new owners have found out how to turn everything on in all configurations and where the dump valves and drains are located and that they work in all those configurations. There are numerous things and I hope your dealer allows you enough time to check everything.
Beware the dealer tech that tries to stop you with, "trust me, it works", or "you'll figure it out the first few times you use it". If something doesn't work, this is the best time to have the dealer fix it NO MATTER how little it is. The dealer will more eager to fix it now and get the rig off the lot, but will try to talk you into "fixing it later" when it's not convenient to you. Do let them know in advance you want to take the time necessary to try everything. This could take 2-4 hours. The more complex the trailer, the longer it WILL take.
Knowing these things will allow you to love your trailer sooner than becoming irritated with it later, because you didn't spend the time you needed when you had the chance! Hope you can get this worked out.
Please realize this is not a protection against "infantile mortality". Forgive the term but it means that something fails sooner than expected (like my fridge that failed 4 months later). But at least when you got it, everything was working.