First, get the result of this command:

assoc .pdf

It gives the current association between .pdf and its document type. In my case, the output is:

FoxitPhantomPDF.Document

Based on this output, I created two .bat files:

ftype FoxitPhantomPDF.Document="C:\Program Files\SumatraPDF\SumatraPDF.exe" "%1"

and:

ftype FoxitPhantomPDF.Document="C:\Program Files (x86)\Foxit Software\Foxit PhantomPDF\FoxitPhantomPDF.exe" "%1"

You have to run the .bat files as an admin.

Sorry, but there is a problem there:

• when I open an administrator prompt in Windows and type the above commands they work fine (!), but when I right click on the bat files and run them as administrator they don't work!

You can use the command 'assoc' (you will need to run the command in Command Prompt as Administrator to change the association). Despite being an old command it still exists in Windows 10. The details: https://technet.microsoft.com/en-us/library/bb490865.aspx

This is a list of methods to stop Edge from taking over PDF. Hopefully one of them will work for you.

Method 1: Using Settings

• Go to Start > Default Programs > Choose default apps
• Scroll down to .pdf
• If "Microsoft Edge" is listed on the right, click it and change to Adobe or else

Method 2: Using Context Menu

• Right-click any .pdf
• Select Open with > Choose another
• Select the desired app, e.g. Adobe Acrobat Reader. If it is not listed, you will need to browse to its .exe file.
• Click OK

Method 3: Using the Control Panel

• Go to Control Panel > Programs > Default Programs > Associate a file type or protocol with a program
• Locate and click .pdf
• Click on Change Program at the top right corner
• Select your desired program, e.g. Adobe Acrobat Reader or .exe
• Click OK

Method 4: Using Registry Editor

• Start regedit

• Navigate to

  HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\AppModel\Repository\Packages\Microsoft.MicrosoftEdge_44.18362.267.0_neutral__8wekyb3d8bbwe\MicrosoftEdge\Capabilities\FileAssociations
  

  (the key might change with future Edge versions)

• Double-click the .pdf key and copy its value, probably AppXd4nrz8ff68srnhf9t5a8sbjyar1cr723

• Go to the following location

  HKEY_CURRENT_USER\SOFTWARE\Classes\AppXd4nrz8ff68srnhf9t5a8sbjyar1cr723

• In the right-pane, create two string values (REG_SZ) named NoOpenWith and NoStaticDefaultVerb with empty values

• Exit the Registry Editor

• Use "Default Programs" to re-associate the file type with your preferred program.

Method 5: Renaming Edge Launcher Files

• Navigate to the following directory:

  C:\Windows\SystemApps\Microsoft.MicrosoftEdge_8wekyb3d8bbwe.
  It should contain the file MicrosoftPdfReader.exe.

• Take ownership of the folder

• Rename the files MicrosoftEdge.exe and MicrosoftEdgeCP.exe (if the latter exists) to some other name.

• Reboot.