Create a new document type? I have some PDF files that I want to open in preview, and others that I want to open in Acrobat. It would be nice if I could just set up that acrobat files with a different extention, say ".apdf" and have the mac automatically open anything with .apdf with acrobat, and anything with .pdf with preview. 

But that doesn't seem to work, because I think that the filetype is actually part of the file, not just the filename...

any ideas?
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.