First, make sure you have the latest version of Adobe (and Flock!) Then please open your Adobe Reader and go to the Edit menu. Choose Preferences and then the "Internet" tab on the left hand side. You should see "Display PDF in browser" please have this checked and hit OK. Adobe should take a couple seconds to configure and once this is done, please close Flock and re-start it.
If you are opening a PDF outside the browser, it will always open in Adobe Reader. The way that Flock's settings work is that if you go to a web page with a PDF file, it should open directly in the browser. In other words, "Display PDF in browser" only works for links to PDFs you encounter while browsing in Flock.
