I tried the Get Info, Open With, Change All, however, when I go to Get Info, Open With and Change All are not there. When I open a pdf a program called Preview opens instead of Adobe. Really need help on this.
First of all you need to select the file that you want to apply it to. For instance, in your case, you would select the PDF file in Finder. If you use "Get info" without selecting the file, you won't see the "Open with" option.
The open with option is a collapsible panel in between "Name & Extension" and "Preview". If you can't see the "Open with" options make sure you click on the triangle next to "Open with"