How to edit a pdf file listed in my google docs.

[roy krausen]

I received a pdf file and transformed it into a google document. Its now listed in my list of documents. I want to edit it. But I can't.  I can't select anything, its just there.  I have chosen to share with someone who sent it so as to collaborate. I am the owner. Surely there's an easy way to edit a pdf file..Thanks.