Hello, my name is Dan.
I will do my best to help you, the only stupid question is the unasked one.
First, do not panic.
Lion has many changes to how it works from before, some applications that ran using Rosetta no longer work because Lion does not support Rosetta.
There is a long list of applications that will not work. You have two choices, stick with Lion and do some updating/upgrading of some applications or revert back to Snow Leopard.
Of the two upgrading is probably the better way to go as it will only continue and you will wind up doing that anyway.
As for PDFs you have built-in to Lion (and OS X before) an application called Preview, the icon looks like this.
It is in Applications. It does a very nice job (much faster than Acrobat) of reading PDFs, it is all I use for this. I create PDFs with Pages and TextEdit or I use the print to PDF.
You can make this default for opening PDFs if you like (I did).
To do that right click on any PDF and from that menu select Get Info...
Like that, then set the default application.
The popup menu where Preview is that is the app for PDFs, then make sure to check the box to make it default.
This works as well in Lion as Snow Leopard.
This is also something you should do, it will save headaches later.