A good Windows forum/help site? Plus pdf is "class not registered" and I don't unders

After many years of Ubuntu-ing, I have now resigned myself to Windows (8.1). This is because I have yet to find a Ubuntu release that will do the old "dual-boot" thing on my UEFI-cursed HP laptop. But enough of that! That isn't why I'm posting.

Now I use Windows 8.1 a lot (I have Ubuntu on one machine, Windows on the other) I'm getting Windows problems; and I don't know of a good Windows users' forum site where I might find good advice.

Can anyone suggest a good Windows forum/help site? Whenever I post a Windows-related post here, it tends to be ignored... even though the forum claims to cover all OSes... So: a good, helpful, nice Windows forum or help site? Please??

While I'm here: There are a number of documents (pdfs) which will open ok on my Ubuntu machine; but if I try to read it on my Windows 8.1 laptop and I can't: the Window2s OS says the pdf is "Class not registered"... which, after a lifetime of Ubuntu-ing, makes no sense to me.

Any thoughts please please people?

EDIT:
I've been using Chrome to read pdfs up til now on my HP laptop, and that has worked well. Until now: the class is not registered or whatever. I have to use Win 8.1's built-in 'Reader' to read the pdfs now, and believe me when I say 'Reader' is crap.

I think I'll have to look for another free pdf reader. Any suggestions?