Finding number of occurences of a word in a PDF file

Inside a folder there are plenty of subfolders and inside the subfolders there are plenty of PDF files. Looks like below:

+- SubfolderA
|  +- File1.pdf
|  +- File2.pdf
|
+- SubfolderB
|  +- File3.pdf
|  +- File4.pdf
|  +- file5.pdf
|
+- SubfolderC
   +- File6.pdf
   +- File7.pdf

I would like to find the number of times a specific word e.g. keywordx occurs in these files. * *Preferably* ignoring to show the files that have 0 hits. * And *ideally* showing the list sorted, with the max number of hits file displayed first.