Editing existing PDF files on iPad

I am currently developing an app that needs to retrieve PDF files from a server, store them locally, edit/annotate them and upload them to the server again. Now the retrieving, storing and viewing of the file is no problem. It's the editing of the file I'm running into problems with.

Now I've looked around and found CGPF/FastPDFKit/LibHaru, but they only seem to provide read-support or only work on MacOS. I've also found that iAnnotate provides an API for developers, but I'd like to explore less costly options before using that.

Do any of you have any experience with editing开发者_开发百科 PDF's or can you point me in the right direction?