Foxit reader and Adobe reader, both are free PDF Readers to view PDF files. If you are using both for some reason, and if you want to set Foxit reader as the default PDF reader, then can set it very easily, follow the instructions below.
1. When you open Foxit Reader for the first time, you will be asked to to set it as default reader, if you’ve not made it as default then, or skipped that dialog,
2. Click on File menu, select Preferences > File Associations
3. Click Make Default PDF Viewer.