While technically you can, this is something I've tried to do without a huge amount of success.
- iframes won't always load a clients web site. This would require the client to modify their site(s)
- PDFs can't be loaded in C3 at this time. C2 had plugins available, but C3 does not at this time.
- SQL Can be written via scripting.
- I had issues getting the screen to look right without having to create forms online.
Simplistically you can write everything elsewhere as web/css, and use Construct 3 as a shell to view it all. I'm not sure how iOS publishable it will be that route however.