Preview as default PDF viewer

I am having difficulty setting Preview as my default pdf viewer. When I open a pdf file with Finder>Get Info>Open with: I cannot choose Preview as it is grayed out. I know Preview is on my computer since I can go to applications and open it manually. Running Mac OS X 10.6.8 (10K549)

Thanks for any assistance.

This is just a stab in the dark.. but have you taken a look at the permissions at the bottom of the Get Info window? I'll post a screenshot of mine so you can compare.

Here is what I have under Sharing & Permissions with Preview Info:

That all looks right to me.. sorry but that was my best guess. I'll re-post here if I think of anything else.

Preview would already be the default unless you have made a change.

What application is launched when you just double click on a pdf to open it?