I should add that this is not the most common problem. If you're double clicking a pdf file in the finder and you want it to open in Preview the solution for that is to do a Get Info (File Menu) or Command I and set the "Open With" section of the panel to whatever you wish, most likely Preview in the case of PaulW.
MisFox is needed when you're opening a file within a browser window and you're not getting the correct helper app.
On my system the
File Mapping for a .pdf file is AdobeResourceSynchronizer and PDF files linked on a web page do indeed open within Safari because the Adobe frameworks have been embedded by an Adobe Reader update.
This is what the MisFox sheet looks like for .pdf files
This is a little tricky because if you click the Choose button next to the Creator field you'll never find AdobeResourceSynchronizer on your hard drive (at least I can't).