Not much info to go on from your post. If you're asking how to create a PDF, then there are lots of tools around these days that will do it for free, including online converters. I think I've used
PDF Converter - Convert to PDF Online Free successfully in the past.
If you mean you want to create PDFs with form fields then any recent version of Acrobat Pro can do it and anyone with version X of Reader (maybe earlier but I can't remember) can enter data into the fields and save it. There will be other programs that will do the same but I'm afraid I'm not familiar with them - but I don't think there is one that is truly free to use.
If you mean that you want a PDF in which anything can be edited......why? One of the early benefits of using PDF for documents was precisely so that it couldn't be edited and many of the current features still seek to offer that facility. If you want people to be able to edit a textual document it's probably better to use MS Word/RTF.
Hope there's something somewhere in the above that helps.