1. Right-click the Start button (or hit the WIN+X hotkey) and choose Settings.

    Settings item in Windows Pro menu

  2. Select Apps from the list.

    Apps button in Windows 10 Settings

  3. Choose Default apps on the left.

    Default apps sidebar item in Windows 10 Settings

  4. Scroll down a little and select Choose default apps by file type.

    Choose default apps by file type link in Windows 10 Settings

  5. Locate the file extension for which you want to change the default program.

    If you're not sure what extension the file is using, open File Explorer to find the file and use the View > File name extensions option to show file extensions.

  6. In the Choose default apps by file type window, select the program to the right of the file extension. If there isn't one listed, select Choose a default instead.

  7. In the Choose an app pop-up window, pick a new program to associate with that file extension. 

  1. Windows 10 will now open the program you chose each time you open a file with that extension from File Explorer.

When you're done, you can close any windows you opened to make these changes.


Method 2

Step 1.   Right Click on the. pdf file to be opened. From the menu displayed, click on OPEN WITH > CHOOSE ANOTHER APP

Step 2 :  Then Select Adober Reader (if listed in the list) and make tick on the check box 'Always use this app to open .pdf files 


**** If Adobe Reader is not available in the list continue to Step 3

Step 3 : Click on More App  and make tick on the check box 'Always use this app to open .pdf files 




Step 4 : Then Click on "Look for another app in this PC"



Step 5 : Then browse for Adobe Reader exe file in C:\Program Files (x86)\Adobe\Reader xx.0\Reader and select the exe file acroRD32.exe


Clicking on the button Open will open the file in adobe reader and will make adobe reader as the default programme for opening pdf files