Will this app suit my needs?

I used to use Evernote, and it was perfect for my use but want to move away from the subscription model.

All my documents are scanned to a searchable PDF, in either OneDrive or iCloud Drive (I can use either)

I presume that the app will support my existing OCR PDF files, and search the text within them?

However, I also need these to be accessible to my wife who has her own iPhone & iCloud account. Is this possible?

DEVONthink can handle PDFs and also has best-in-class search capabilities.

You can’t sync between accounts (especially iCloud). If you need collaborative syncing with a remote sync location, you’ll need to use a common account on another service like Dropbox. Otherwise a direct connection (Bonjour) can be made between devices when they’re on your local network.

if you have lots of OCR‘d PDFs than DT2Go is perfect, because the annotation tool is great, much better than Evernote.
Unfortunately there are some feature still missing, for a long time, like save searches for re-use, no smart folder sync DT / DT2Go - and taking notes is a pain, if this is an important part for you you should consider to use a 3rd party App (e. g. Pages) and store those notes in DT2Go - unfortunately you cannot open those files directly in DT2Go - you have to go through the Files app.

So my conclusion is: Evernote is great to take notes (and much more) and DT2Go is great to organize (existing) documents.