I'm glad to inform that you can easily set PDF Reader Pro for Mac as a default program to view files. To do that please follow the next steps:
1. Open Finder, click the pdf file and hit Command + i；
2. Or choose File on the main menu；
3. Or single click the file with your two fingers;
4. Or right click with your mouse;
5. Then choose Get Info > Open with > Select PDF Reader Pro for Mac > Change all....
If there are different versions of PDF Reader Pro in your device, please uninstall All first, then redownload and install the right version.