Since you already know all the files that are PDF's but don't have the extension, you should move them all into the same folder and then add the extension to all of these files using a renaming script or something of the like. this would be much easier than trying to explain to your computer that you want everything without a file type to open in a PDF viewer.
You really want your computer to ask you every time you open a new type of file or a file with no type, other wise you will come to an instance where you have a bunch of files that open in your PDF view and you don't want them to, and you would then have to figure out a way to reverse what you have done for this specific file type.
Just add the extension to these files, it will save you a headache in the long run.
ren *. *.pdf
?