Run the cmd and execute: Dism.exe /online /Export-DefaultAppAssociations:C:\Temp\DefaultApps.xml
Open the .xml file and edit lines related to pdf files (for example see code below):
Code:
<?xml version="1.0" encoding="UTF-8"?>
<DefaultAssociations>
<Association Identifier=".acrobatsecuritysettings" ProgId="AcroExch.acrobatsecuritysettings" ApplicationName="Adobe Acrobat Reader DC" />
<Association Identifier=".fdf" ProgId="AcroExch.FDFDoc" ApplicationName="Adobe Acrobat Reader DC" />
<Association Identifier=".pdf" ProgId="AcroExch.Document.DC" ApplicationName="Adobe Acrobat Reader DC" />
<Association Identifier=".pdfxml" ProgId="AcroExch.pdfxml" ApplicationName="Adobe Acrobat Reader DC" />
<Association Identifier=".pdx" ProgId="PDXFileType" ApplicationName="Adobe Acrobat Reader DC" />
<Association Identifier=".xdp" ProgId="AcroExch.XDPDoc" ApplicationName="Adobe Acrobat Reader DC" />
<Association Identifier=".xfdf" ProgId="AcroExch.XFDFDoc" ApplicationName="Adobe Acrobat Reader DC" />
<Association Identifier="acrobat" ProgId="acrobat" ApplicationName="Adobe Acrobat Reader DC" />
</DefaultAssociations>
Open cmd again and run: Dism.exe /online /Import-DefaultAppAssociations:"%~dp0DefaultApps.xml"
... to be honest, I am not sure if these will work because changing file associations, the same as inserting entries in the mouse context menu, is one of the downfalls for the portable apps. Not always it depends on producers too...