>> What do I put in the button's URL field in order to display that file and/or what do I need to do in advance to be sure that Adobe Reader or other viewer is available?
let's do this one first. Certainly from the server side there's no way to know off-hand if the PDF-Plugin is available. I suppose you might create some JavaScript to detect it - I don't know if that's possible - but it would be prone to failure as browsers change, as new (possibly mobile) browsers appear and so on.
On the other hand if the plugin is not available the browser will offer to open the file "in the registered program for that file type" - meaning it'd open in a PDF reader outside the browser. If there is no PDF reader on the machine, well, I've not bothered myself to go there. I think PDF is sufficiently wide-spread that people either have a reader, or have someone they can ask about getting a reader.
So in short, personally, this falls into "not my problem" - it's the user's problem, and one I'm guessing in the vast majority of cases has already been solved.
>> My question is how do I load it to the new URL.
there's a URL field for the button which you can set. All you need is the name of the file. (assuming the pdf is in the web folder) or a relative pathname if it's in some sub folder.
cheers
Bruce