Step 1: You need to left click on the pdf file you want to change the default opening program.
Step 2: Now you will need to right click to access the menu items for that file.
Step 3: In the resulting pop-up window, you will go to the Properties listing and select it with your left mouse button.
Step 4: The Window below will show the properties where you can select the Change button.
Step 5: In the window that appears, you can select the program of your choice. (while I use a web browser, you may want to Adobe Acrobat Reader DC)
Step 6: Once you make your selection, click on the Ok button confirming your selection.
Step 7: In this screen you will need to click on the Apply button making your selection confirmed.
Step 8: Once you close this box by clicking on the Ok button, your pdf files will now open with your preferred application.