Sayan already provided the solution, but maybe you missed the relevant point. I can try to rephrase (we assume you're using the default GNOME desktop manager)
1) Right-click on a PDF file and select Properties
2) Switch to the Open With
3) Select the desired default application from the list
From now on, all the PDF files will be open using the selected application. If you prefer to set-up things from the command line, here
is a good starting point. In addition you can check the official Red Hat and/or GNOME documentation to find out.