In Mac OS X, Apple's Preview is the default application for opening a number of file types, including PDFs. While versatile, Preview will NOT allow you to make changes to PDFs with editable content such as forms. Follow these instructions to set Adobe Reader as the default application for opening PDF files.
Single click a PDF file you wish to open or any PDF file in general while holding the control key. Then select 'Get Info' from the menu you've opened.
Under the heading 'Open with,' the current default application for opening PDFs is listed. If there is no application listed you may need to click the triangle to the left of 'Open with' so that it points downward to reveal the current setting.
Click on the application listed to open a menu displaying additional applications. In this menu locate the most recent version of Adobe Reader. If Reader is not listed or an old version of the software is listed select 'Other' from the bottom of the menu. Navigate through your Applications folder to locate the icon for the Adobe Reader application and click to highlight it. Click the Add button to select Reader.
Back in the information window click the 'Change All' button so that all subsequent PDF files will be opened by Adobe Reader.
Click the Continue button to confirm that you'd like to change your settings. Now all PDF files will be opened by Adobe Reader.