When it comes to opening PDF's Adobe has cornered the market but here are some great alternatives to use without using Adobe.

Fox It Reader is a great lightweight tool for reading PDFs and it's free

Cute PDF is great for creating PDFs and reading and it's free

Finally remember if you have a Google Account you will be able to open PDFs in G Mail.

Adobe has had a few security issues lately so it always pays to check for an updated version and do not turn off the automatic update checker.