The standard for opening PDF files has been Adobe Acrobat reader. I've been using this for many years and have always been underimpressed with it. The main problem for me is load time. When I click on a PDF link from a webpage I want the document now, I don't want to wait 10-20 seconds. It's so slow that Google offers to convert pages to HTML so users don't have to feel the pain of the PDF.
Recently I found a great replacement called FoxIt reader. At first I thought it was Acrobat but it's in fact a look-alike that loads PDF files in about a second. It seems to have all the same features without all the bloat. PDF files open on screen in less time then it takes for Acrobat to display it's initial splash screen.
It's a nice little application, FoxIt could do to Acrobat Reader what FireFox is doing to IE. As it stands FoxIt has a clone UI and it's website doesn't exactly look professional. This makes it a follower not a leader. If it took some steps to differentiate itself it could really take off as the new leader for opening PDF files.