title_body: How to sort on books that aren't in a specific format in calibre
upvoted_answer: I'm surprised nobody has responded in six months, sorry I haven't been here :-(
  
  I do this all the time.  
  
  You want to convert everything that has formats:"=PDF" and doesn't have formats:"=MOBI".  You can just put those conditions in the search box, or in the tag browser:
  
  
  Select the PDF format (which will place a big + beside it), then
  ctrl-select the MOBI format twice, instead of a + there'll be a -
  
  
  You'll see the following selection string in the search box: not formats:"=MOBI" or formats:"=PDF". Now you have all the books that are either not MOBI or are PDF.  That's not nearly what you want, but now just change the or to and (or delete it—no conjunction is the same as using and) in the search box and you have all of the PDFs that don't also have a MOBI.