You can change the file associations in Finder or in ACDSee Pro for Mac. Following are the examples to change the PDF file association to Preview.
In Finder (recommended):
1) Select PDF file and open Get Info pane - Command+I is the shortcut
2) In the Open With section select the application, e.g. Preview
3) Click Change All button
4) Now all PDF files are associated with Preview
In ACDSee Pro for Mac:
1) Select PDF file and click Properties to open Properties Pane, then select File from the drop-down menu.
2) In the Open With section select Other...
3) In the Choose Application dialog select Preview
4) In the Choose Application dialog select “Always open with” and “Apply to all documents of the same type” check box