Create file association i registry

You dont have to access the registry for that.

Method one:
Right click on a PDF file and from the menu select 'Open with...'
Choose the program from the list and check the box at the bottom of the window which says something like 'Always use this program to open this filetype'. Then click 'OK' and it gets applied to all PDF files.

Method two:
Uninstall Adobe Reader (if you have more PDF programs, remove them too). The file extension will then automatically be opened by PDFViewer.

Well, i have to change it on 3000 machines... so i have to deploy the changes

There are two other methods:
1. use ASSOC (http://ss64.com/nt/assoc.html)
2. modify this registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.pdf\OpenWithList\

Registry method: modify this key HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.pdf\OpenWithList\

And another method (not my idea and I want to give credit to rejoinder for it):
Open the "Group Policy Management" snap-in.
Traverse to the correct OU and create a new policy.
Edit the policy and open the editor to Computer Configuration > Preferences > Control Panel Settings > 
Folder Options
Right click "Folder Options" and pick New > File Type.
Select "Create" from the drop-down box.
Select "File Extension" from the "File Type Settings" drop-down box.
Enter the file extension for the new group policy.
Select a program from the list of software in the "Association" list.
Click "OK" to save your settings and close the group policy editor.

You now have 3 different methods.

Here's a couple of options.  One uses the registry, the other uses FTYPE and ASSOC DOS commands.
https://www.experts-exchange.com/questions/22894965/Associate-tif-and-mdi-files-with-Microsoft-Office-Document-Imaging.html

Rob.