I've been using the PN-40 since it was first available. I have a few suggestions, then some thoughts on learning.
First, I prefer to use Lithium batteries which last much longer. I always carry two extra sets of batteries. Then go to the DeLorme website and buy the map subscription for $29.95 per year. The US topo map disks that come with the PN-40 are good for many things. I also download USGS topos and color aerial images for hiking is new areas. While you are at the website join the forums - there are some very knowledgeable people with answers to nearly any question. Register your PN-40 so you will have the warrenty, assuming it is new. Once registered you will also get notices about firmware updates.
Read the manual to get an idea of what the PN-40 can do. Now you should have some thoughts on how you want to use it. I don't think most manuals are much good for learning how to do what you want. The PN-40 is the seventh handheld GPS I've owned, the first five were Garmins.
Go outdoors and walk around close to home while practicing the things you want to do. Some people like geocaching(sp). Some hikers just turn on their unit whan they want an exact position. I keep my GPS turned on all the time I'm hiking. I want to know how far I hike each day. I set the distance back to zero each morning. I mark the location where I park my vehicle. I mark the location where I camp each night. Then I write comments in my notebook about the campsite.
The only way to learn to use a GPS is to practice. All GPS receivers do the same thing. The programming (bells and whistles) are different with each make and model.
The menues are a little different with each screen. When I first got my unit I made a list of what was on the menue from each screen.
Just my thoughts,