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.