This is a really interesting question. I was aware of several paid applications that allow you to do this but I really want to find one that was free to answer this question and I did come across a really simple useful way to convert from read only P.D.F. to a read write format and the answer is to use Google Docs.
Google Docs is free if you have a gmail account you can just log into your account go to Google Drive. There upload the P.D.F. file and then open it in Google Docs and Google does the job of converting it from just a picture of text into real editable text so if you don't remember how to do this from listening on the radio/
One easy way to do this is search for convert P.D.F. to text Google Docs and you'll find an answer how to do this online for free. Once you've made the edits you need to make in Google Docs is just a word processor on line.
You can then save it again as a P.D.F. file and send it on its way. I'm very excited about this free technique using tools that I already use.