Create a new document type?
Select the file in Finder, then get info (cmd-i), and change the file extension (under name and extension) to something that is not used by other files.  

Then, near the bottom of the get info window click "Open With" and select the app you need, i.e. Adobe Acrobat, by browsing through your application list.  Then, at the bottom of that sub-pane click the 'change all' button.  This will direct Acrobat to open all files with the extension you chose.