WINDOWS USERS
Method 1
Right-click on the PDF file, choose Open With > Choose default program or another app in.
Choose Adobe Acrobat Reader DC or Adobe Acrobat DC in the list of programs
Select Always use this app to open .pdf files.
Note:
If you have both Adobe Acrobat DC and Adobe Acrobat Reader DC installed on your computer, choose Adobe Acrobat DC.
Click OK
Method 2
Click on the Start menu and start typing Default apps.
Click on that option when it appears in the list.
On the right side of the window, scroll until you can see & click on the text link for Choose default apps by file type.
On the right, locate the hidden scroll bar and scroll down until you see .pdf in the very left column.
To the right of .pdf, click on the shown option (probably Edge).
From the drop-down list select either Adobe Acrobat Reader DC or Acrobat DC.
You can now close the Settings window.
MAC OS USERS
Click the PDF file's icon in the Finder to select it.
Choose File > Get Info.
Click the triangle next to Open With, and choose Adobe Acrobat Reader or Adobe
Acrobat from the pop-up list (if your preference is not on the list, choose Other to select
Click Change All.