EagleFiler is basically a competitor to Devonthink/KeepIt/Notebooksapp. It’s been around since 2005 and was written by the Mac dev of the longstanding SpamSieve app. He keeps it up to date, regularly updating it, and is in the site’s forums when people have any questions.
It acts like a simpler version of DevonThink Pro, which is the I switched from (I wasn’t crazy about the UI, or the customer service, or a longtime, repeatable bug they only finally fixed a year after I dropped the app).
If you like it you might also consider those three other apps, all of which offer iOS versions and syncing. But I still use and recommend EagleFiler as a Mac solution.
Neither the Files app nor the Dropbox app will cache content locally by default.
In Dropbox you can mark files and folders to be cached for offline storage, but I still think the updating may only happen when you open the app (background updating has never been a strength of 3rd party apps in iOS).
I can’t speak much to the way that iCloud Drive works, because I use Dropbox for anything I really care about.
If you do want to cache more files on your iOS devices, I would recommend checking out Secure ShellFish “SSH and SFTP support in the iOS Files app” which is a new app which I’ve been trying out which has “Easier to access than cloud storage” as part of its tagline.
I downloaded it the other day and immediately paid for the temporarily-discounted IAP soon after. It got a lot of attention last week after launch when Ryan Christofel wrote about it.
Like others on here, I tried Evernote for a while, but grew frustrated that all of my scans - along with many other notes - were tied to a proprietary bucket that I was paying for. In the end, I made the painstaking choice to migrate everything back to Dropbox. I not-so-fondly remember one summer where I exported all the Evernote notes in HTML format, and slogged my way through creating shell scripts to clean up the mess of HTML-formatted text notes, images and so on into a simple, clean hierarchy of directories.
In short: nothing beats PDFs (or image files, if you’re so inclined) in a file system! Automation options aplenty!
I am still in Evernote. It does the job and I’ve built muscle memory around it. The full-texts search is great. I have though about moving to something else (I don’t love the idea that I am locked into a proprietary system), but haven’t had the need to move off it.
I am spending more and more time on the iPad vs. the Mac and that make a huge difference in ease of use with Evernote.
I use DEVONthink as well but I try to cut down my app usage and find an app that can do both extensive notes on Mac and iOS, and secure filing. I have been toying with Keep It which I find very interesting for that purpose. You can also encrypt files in the app, removing the need for external solutions and it all works with iCloud. I recommend taking a look at it.