PDF is a WORM format, as far as I'm concerned.
We use it in work to say "This is it, this is the document, this is how it looks, nobody change it" and then offer that to customers knowing it will look the same no matter what they open it on and it can't be tweaked. Yes, we know you *can* edit them, but you can't edit them easily or nicely or guaranteeably.
Draft in Word, publish in PDF.
It's a great format for that. This is the version, no changes. Sign it if you have to. Beyond that, it's really just another format.
I refuse to buy Acrobat, though. I paid for Nitro once when it was cheap and that serves all my needs. For years (and still currently), I used PDFCreator and other freebie Ghostscript-based things to create PDFs if I needed them.
I don't see that the format needs much extension.
However, I was recently asked how to "stop people stealing our pictures out of PDFs" (and also website images). My solution was "don't put them in there" because you can't beat an analogue hole (screenshot tool) and PDFs you can suck the content out any time you like. They can't restrict "reading" permissions.
The biggest problem with Adobe is all the plug-in shite that tries to put such limitations and other DRM on you. I have one that literally interferes with EVERY PDF you print by watermarking it, whether or not it was part of the purchased PDFs that had that DRM. We stopped buying that stuff, fortunately.
Keep it to a display format. I mean, use the forms stuff if you have to but even that's a security risk (running Javascript and talking to outside websites, etc.). Anything more is really a nonsense and won't be used and will contribute to the long-term death of the format.
PDFs are fine. I mean anything would be fine, but XPS. But Adobe can't be making much money out of them at all.