Find a PDF on your hard drive. Right click it (or just click it) and select "Get Info" (or click File>Get Info or press CMD+I)
In the Info window under Open With, choose Reader 9 and click OK. THis should set Reader as the default to open PDFs.
In your browser you need to enable the Reader plugin either in the preferences (Safari) or under Add Ons (Firefox) and then the PDF's should open.
If not, your Reader install may be corrupted.
If it is, downlaod a trial of App Delete and install it. Then open it.
Drag the Reader app from your Applications folder to the AppDelete window.
Delete everything in the window, and empty your trash.
Redownload and reinstall Reader 9.
Things should work normally.
What I ended up doing was getting rid of Adobe Reader 9 piece by piece by piece. It simply would not open a pdf in the browser. Kept giving me a message window saying I needed to "find" an application to view it, yet there it was visible in the window already installed. This happened with both Safari and Firefox. I checked preferences, it was set correctly to open pdfs in browser. There's a "repair installation" function, tried it, said it didn't need it. Then i found long discussions in the forum from other users who were having same problems of not being able to view pdfs in browser with AR 9. They went back to AR 8. So that's what I did too. Whole thing cost me about 3 hours though. Argh.